Gently used?
You can pop an internal ATA/IDE CD (or DVD) burner into an external case (saw a USB2, Firewire combo case for $45 new!).
Just make sure it has a removable plate to allow the tray to pop out.
Check out XLR8 your mac for real-life reviews.
This is can be way cheaper than any pre-built - if you buy a used internal unit and get an on-line case (see DealMac).
OR, wait for an Office Max, Staples, CompUSA "deal" for a new burner.
What you'll need to be careful about is getting a drive that will be recognized. Toast should be able to do the trick for you though.
Good Luck!