Most of the places around here charge between $5 and $15, however a lot of times, they'll say "don't worry about it this time. Just keep us in mind next time you're shopping for tires."
For what it's worth, you can get a plug kit (3 or 4 plugs, cement, tool) for $1.99 at Walmart. However, your tire warranty may be voided if you use them -- some of the warranties (at least my current one) states that leaks need to be repaired from the inside (or something like that).
Regardless, I found out that it's easier to plug the leak (if you can find it) than it is to take the tire off and put on a donut.. especially if you're 100 or so miles from home and are faced with driving <50 mph for a long duration. Just pull into a convenience store, get a plug kit (they often sell them there) and a glass of water. Pour the water over the leaking tire to try to locate the leak. Plug it. Re-inflate tire to the proper pressure. Done.