Does the dealer look it up by VIN or by the serial number off of the back of the radio? Either way, the dealer shouldn't charge you for giving you the code as long as you provide the information (if they have to remove the radio then yes, there will be a charge). If they try to charge you for the code when you have the VIN (or serial number), just tell them that you will remember this the next time you need work on your car and then call a different dealer.

But let's look at the big picture here - you can buy a really nice aftermarket radio for $150 or so from Crutchfield, one that plays CDs (!), .mp3s, .wmas, .aacs, and might even have a front USB port. Installing it is very easy though you may or may not need to bypass the external amp (located under the driver's side dash), not a big deal if you do.