Here's some used car prices from What Car? online...
Lexus IS Saloon 200 2.0 SE 4dr 2003 (03) 23,000 miles
Dealer £6,540
Private £6,160
Part exchange £5,725
Trade £5,470
If yours is an auto you can probably add a few hundred to the price
Also, try Webuyanycar.com - I tried but you have to put in your reg.