Hi m8. There are quite a few is250's out there for sale and at really low prices dependant on spec, service history, number of owners etc. You could get an idea of your cars worth by going on we buy any car. Just give a dodgy email address and location.
When I got mine just a few weeks ago prices ranged from £1k to 6k. Mines an 07 plate SE-L with full lexus service history and one owner. 5.5k I paid. Good luck in your hunt.