Q. When you factor in fees and finance charges, how much does a "free" airline ticket, acquired by redeeming credit-card frequent-flyer miles, cost?

A. For consumers who charge with abandon and pay off their balances in full every month, travel-reward cards, which typically grant you one frequent-flyer mile for every dollar that you charge, can be a great deal. For just about everyone else, however, acquiring free plane tickets through these cards is a mighty expensive way to fly.

Here's why: The expenses associated with airline and other travel-reward cards are higher than those on standard cards — annual fees average $50 a year (standard cards typically cost nothing), while finance charges can run a couple of points higher (most are currently 15 to 19.8 percent compared with 9.9 to 17.75 percent for competitive standard cards). If you pay off your balance in full every month and charge about $12,000 a year — the average amount charged on airline cards last year — you'll earn a free round-trip domestic airline ticket worth approximately $500 in about two years, figures Robert McKinley, CEO of CardWeb, an online credit-card research service. Since you'll have shelled out only $100 in annual fees over that period, that's a good deal. But if instead you charge only, say, $3,000 annually, you'll need at least eight years to earn that same ticket, and will shell out about $400 in fees along the way. So in the end it's a wash.

Carry a balance on that airline card, however, and the real price of that free ticket rises dramatically, McKinley notes. Say, for example, you charge around $4,000 a year (the average amount purchased on nonreward cards last year) and carry a typical balance of $2,500. Assuming the airline card carries an 18 percent interest rate, you'll earn a free ticket in six years, but will have paid $300 in fees, plus around $2,700 in interest along the way. Total outlay for a ticket worth $500 — $3,000.