Yep the prices are defiantly on the up as you don't see them on sale too often, and 10k for a good original example does sound about right in recent searches. The price really does depend on many factors such as millage, documented service history, and weather it is original or not. Expect to pay top dollar for an nice, clean original example.
Always check and make sure it is a factory G60, as there are many converted G60's about.
Check for charger history, but don't let it put you off if it hasn't been done, as there are quite a few places about that do rebuilds for about 350 quid. Just negotiate harder if it hasn't been done.