QUOTE(kam05 @ Jun 25 2008, 02:57 PM)

WhatCar reckons your GS300 Sport is worth anywhere between £4k (trade) and £4.6k (private) with that kind of mileage, although I suspect the true/actual selling price might be slightly lower than £4k- reason: not many people want to buy/run a big 3.0 litre car with the current price of fuel/tax.
What's the reason for wanting to sell, if you don't mind me asking? If there is no real reason to sell (ie: I fancy a new car), I suggest you might as well as keep it (if you can afford to pay for it, fuel/tax/insurance/repairs etc) because you have a quality/reliable motor. If your circumstances have changed (ie: need a diesel because you are driving long distances or you need a bigger car (MPV) for the family), then IMHO that is a valid reason to change cars.
I'm exactly in the same situation as you, I have considered selling my IS200 (on a 2000 'X' plate) with 107k on the clock recently. With that kind of mileage, my car is worth £3.5k-£4k (according to WhatCar), I think if I actually came to sell it, I'd probably get £3k to £3.3k if I'm lucky. So I've decided to run my car until it falls apart, besides, as my car is registered before 1st March 2001, my road tax will cost me £185 next year, instead of £400 (for all post 1st March 2001 reg'd IS200's).
It's a bad time to be selling a big petrol engine car, sorry.
Having owned a GS300 Sport Series 1 for 6 years and driven IS 200 on several occasions I found very little difference in fuel consumption. As for road tax If the 300 is on an " X " it may well be before March 2001 the same as yours meaning that the tax would be exactly the same. I don't think that this is really a bad time to be selling " a big petrol engine car " as I have (4 weeks ago) changed my '96 300 Sport for a 52 reg 430 SE Nav and I can't believe that I am the only person buying this type of car.
NB As regards fuel costs, I have so far found the 430 to return very similar consumption to the older 300 Sport, 17/19 town use, 28/30 on run at "around" 70ish.