When is a free ticket not free? When you get it through your airline's frequent flier program.

For a little perspective, think of these loyalty programs as a business (and some financial analysts have even suggested some of these programs may be more valuable than the airlines themselves). Yes, you earn miles or points, you get trips: In 2012 alone, American Airlines redeemed over 5 million free one-way trips.

But there are some gotchas.

What does a free ticket cost?

The main gotcha is that you usually pay something for your free trip, one way or the other.

For example, to amass the 25,000 miles typically needed for a free domestic round-trip flight, you personally have to spend over $2,000 on airline tickets (that's a rate of eight cents a mile). That's not a great bargain unless you're flying mostly on your boss's dime.

Or maybe you don't have enough miles or points so you decide to buy some from your favorite airline; the typical rate is over two cents per mile or $500+ for a domestic airline ticket.

Here's another catch: In most cases, you will struggle to find award seats available on your preferred days to fly to the most popular destinations, and there are usually are plenty of holiday blackout dates.

READ MORE: Don't let blackout dates ground you

Another catch: Surcharges, taxes and fees

Still, free is free, except when it comes to taxes and redemption fees.

Taxes and fees aren't so bad for domestic flights, maybe averaging about $5 to $50, but you can really get socked redeeming miles for international flights. British Airways, for one, says its rewards members are liable for "all taxes and other charges including without limitation, airport departure tax, customs fines, immigration fees, airport charges, customer user fees, fuel surcharges, agricultural inspection fees…" and so on.

The biggest gotchas on international flights are taxes and fuel surcharges. You can shell out as much as $1,000 for a "free" flight depending on where you're going, and even on a U.S. discount carrier like Frontier, you may pay as much as $100 or more in taxes on certain trips to the Caribbean and Latin America.

MORE: How to shop for flights to Europe

If you're a procrastinator, there's a fee for that, too. For example, American Airlines charges $75 to redeem your "free" ticket if you do it within 21 days of departure.

And then there are the baggage fees: You generally still pay those unless you're redeeming a lot of miles for a first or business class trip. And please, don't redeem by phone! Yes, another fee.

Then there's the matter of what your time is worth. As I mentioned, it's often hard to use miles when you want to, particular to popular destinations at peak travel time and redeemers must learn to juggle schedules.

Yes, you do get something for free - but it's probably not quite as free as you thought.

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