How much its worth will be based majorly on the following (assuming you're lvl70):
Your class
Your gear
The quality of your battlegroup
If you're not lvl70 don't expect to sell the account as anything more than you would a 2nd hand game. As a rough idea though, I recently sold my 70Paladin with full T6 (pre-2.4) for £420.
If you sell it, the CD-Keys for both TBC and the original should be part of the package along with the account information, including your secret answer. Before doing so go into your account settings and remove any personal information you have there, if the password you use for your WoW login is the same as anything else (hotmail / amazon etc) then change it.