I have a simple and not particularly edifying answer: Who the heck knows?

For years, the conventional (read: wrong) wisdom has been that travelers should value their miles at 2 cents each. There may have been a time – say, back around 1984, when I had hair and could claim two free unrestricted first-class tickets to Hawaii for 75,000 miles – when 2 cents a mile was a practical figure. But not lately. Not for years.

In truth, frequent flyer miles have no intrinsic value. They are only worth the equivalent of what you’d pay for a ticket that you can claim with a frequent flyer award. In other words, miles are tied to fares, and the moment you bring fares into any equation, chaos ensues.

So I did what we all should do: homework. I played around with some routes, some dates, some destinations and some fares from each of the Big Six carriers. I compared both restricted and unrestricted miles, advance purchase and walk-up fares, domestic flights and international itineraries. The range of “value” I discovered is surprising.

And without working too hard, I quickly came to some clear conclusions. Cash in for low-priced tickets and your frequent flyer miles could be worth just a fraction of a penny. Get lucky – or, in my case, get a tip – and your miles could be worth twice the old 2-cent rule.

I present the following as a guide and as an example of how you should approach the valuation of your miles. What I found below is useful, but it is not authoritative because, well, there is no authority here.

American Airlines
Let’s say you’re sick of the snow and the cold in Chicago and you decided to take a little vacation to San Diego. This afternoon I was able to book an Easter holiday trip (departing March 21 and returning March 29) in coach for $198.40 at aa.com. American also offered the same trip for a restricted AAdvantage award of 25,000 miles plus a $5 payment for the September 11th security charge. Which means you’re trading 25,000 miles for a cash savings of $193.40.

Bottom line: One American AAdvantage frequent flyer mile would be worth .77 cents. For those of you flummoxed by the decimal point, that’s slightly less than eight-tenths of a cent per mile.

But let’s say you need to get to San Diego tomorrow and want to return on Monday, February 28. I booked a coach seat this afternoon on aa.com for $418.40. Surprisingly, aa.com would also book the itinerary for a 25,000-mile restricted award. Of course, the “free ticket” also carried the $5 fee and American’s $75 charge for booking a freebie within 21 days of departure. Which means you’d be trading 25,000 miles for a cash savings of $338.40.

Bottom line: One American AAdvantage frequent flyer mile would be worth 1.35 cents.

Okay, but let’s say you just gotta get out of the cold tomorrow and only a first-class flight to San Diego will do. This afternoon I booked an American itinerary in first-class with a return on Monday for $1,118.40. aa.com did have a free first-class seat, but only for the 90,000-mile unrestricted AAdvantage award plus the $80 in fees. Which means you’d be trading 90,000 miles for a cash savings of $1,038.40.

Bottom line: One American AAdvantage frequent flyer mile would be worth 1.15 cents.

Continental Airlines
Continental’s Newark hub happens to be my closest large airport, so let’s say I want to fly to Cleveland to catch a few Indians games at Jacobs Field on the first weekend that the Tribe is home during the 2005 baseball season. If I’m willing to fly obscenely early (6:40 a.m.) on Friday, April 15, and return just as early on Monday, April 18, Continental will give me a coach seat for a restricted award of 25,000 OnePass miles and a $5 fee. While I was searching for this award, continental.com conveniently alerted me to the fact that I could buy the seats for $156.90. Which means I’d be trading 25,000 miles for a cash savings of $151.90.

Bottom line: One Continental OnePass frequent flyer mile would be worth .60 cents. That’******s six-tenths of a cent per mile for those of you with a decimal problem.

But let’s say my wife wants to take me to Edinburgh for my birthday later this year. After all, we had a great time in Scotland last May on a trip put together by Travel Insider David Rowell. And say she’s willing to splurge for a business-class ticket because I’m such a sweetheart. Continental is running a business-class sale just now, so I was able to book a Newark-Edinburgh itinerary (departing Monday, May 16, and returning Monday, May 23) on continental.com for $2,217.65. Should she wish to fly me free – hey, I didn’t marry no fool – she’ll have to come up with 150,000 miles and $116.63 in taxes and fees. That’s because continental.com says only unrestricted-level business-class seats (100,000 miles) are available going to Edinburgh, but restricted-level seats (50,000 miles) are available on the way back. All of which means that my loving wife would be trading 150,000 miles for a cash savings of $2,101.02.

Bottom line: One Continental OnePass frequent flyer mile would be worth 1.4 cents.

Delta Air Lines
Let’s say you’re headed from Atlanta to Phoenix on Thursday, March 10, and want to return on Thursday, March 17. Why? Well, no reason other than those are the days that delta.com displayed when I surfed there today. A roundtrip in coach cost $318.40. With a little fiddling, I was able to find a restricted-level award for the same days for 25,000 SkyMiles and the $5 fee. Which means you’d be trading 25,000 miles for a cash savings of $313.40.

Bottom line: One Delta SkyMiles frequent flyer mile would be worth 1.25 cents.

Northwest Airlines
I think by now you’re getting the picture that the value of frequent flyer miles is almost totally dependent on the class of travel you choose, whether you can cash in at the restricted or unrestricted award levels and what the airlines feel like charging at the moment you book. So let’s pick up the pace a bit. I went to nwa.com to research the matter, and it conveniently offered a form with a pre-chosen departure of March 3 and a return of March 10. So I entered DTW (for Northwest’s Detroit/Metro hub) and NRT (for Northwest’s Tokyo/Narita hub) and nwa.com returned a nonstop roundtrip flight for a coach price of $818.47. I duplicated the parameters for the trip using Northwest WorldPerks miles, and back came an offer of seats for the restricted 60,000-mile award plus $71.47 in taxes and fees. Which means you’d be trading 60,000 miles for a cash savings of $747.

Bottom line: One Northwest WorldPerks frequent flyer mile would be worth 1.24 cents.

United Airlines
Of course, it’s easy to change the entire game with one savvy bit of planning – or a little inside information. Earlier this week a Denver-area travel agent tipped me to the fact that several of her clients had just scored last-minute, first-class flights to Hawaii using only 60,000 Mileage Plus miles, the restricted level. So off I went to united.com today. For a departure tomorrow with a return on Friday, March 4, United was selling first-class seats on its Denver-Honolulu nonstops for $2,442.30 roundtrip. But as the travel agent suggested, united.com was also offering first-class seats for just 60,000 miles and the $5 fee. Which means you’d be trading 60,000 miles for a cash savings of $2,437.30.

Bottom line: One United Mileage Plus frequent flyer mile would be worth 4.06 cents.

US Airways
That four-cents-a-mile figure is eye-popping, but, as you’ve probably guessed, an anomaly. A little knowledge can also help me “prove” that frequent flyer miles are worth virtually nothing. Take a flight to Orlando from Philadelphia, where US Airways is in a life-and-death struggle with Southwest Airlines. I know fares there are at record lows. So at usairways.com today, I priced a roundtrip departing Thursday, March 15, and returning Thursday, March 22, at just $176.90. Usairways.com also reported that the same itinerary was available for the restricted level of 25,000 Dividend Miles plus the $5 fee. Which means you’d be trading 25,000 miles for a cash savings of $171.90.

Bottom line: One Dividend Miles frequent flyer mile would be worth .68 cents. Again, for the decimally deficient, that’s less than seven-tenths of a cent a mile.