Before answering your question I will add the caveat that without seeing your problem and without knowing the exact spec of the bike, this 'quote' could be wrong. For one, you might not have replaceable chainrings. But, assuming you do have replaceable chainrings the cost would be somewhere around 30-35$ (10$ labor, 20-25$ for chainring) depending on the quality of the chainring (shift ramps/machining/material). It could be much more if you were to be using something like Shimano XTR or SRAM XX.
A way to see if they are replaceable is to inspect the crankset and find either chainring bolts holding the chainrings on or rivets holding it together.
If it is riveted the shop might use a slotted screwdriver or a set of channel locks, if possible, to bend the chainring back into place. This depends on the amount of clearance provided by the other chainrings. If that is not possible then they might have to remove the crank arm and hammer it straight using a mallet. With those possibilities in mind it might run you 10-20$ for the fix.
When going to your LBS be sure to identify the issue you want solved and express that you simply want that one problem solved. Often the reason a problem fixed seems more expensive than it should is that the mechanic has to spend the time IDing the issue. Time is money in any business. By identifying the problem right away you have saved time and therefore money. Be sure to get a quote before leaving the bike.
I believe the cause for your bent chainring has been addressed already. Starting from a half shift or shifting under load can cause this. The less expensive the drive train the more likely it is for this to happen. A riveted crankset w/ stamped chainrings will bend quite easily while a more expensive crankset with replaceable chainrings that are not of the stamped variety will be more resistant to bending.
As a note, I am in Canada ... therefore this prices might not be entirely reflective of the prices in your specific area.