I believe the top (condiotion) price will be in line with international prices. In SL there's no mkt as such.... A few guys have a load of cars. Let's say you wake up tomorow and decide to by a Caddy. What can you do about it? Not much. Just keep your ear to the ground and wait until someone decides to sell. Which is so unlikely as the cars are with well heeled guys. You can't tempt them with an above average price. And you really have to be crazy about the car to do that. Where as a resto poject will be higher than the international prices. The seller's probably selling because he can't afford to restore it or has lost interest, needs money etc. And restorartion eventhough a lot harder in SL than in the UK is still a lot cheaper considering the labour rates. So it you can pick it up higher than the UK price and still bring it to concours cond with your cost lower than the UK price for a car in that condition.
But buyers (and sellers) do need a reference and usually try to stick by the UK prices.