5-10 years ago you could barely give them away. Prices have increased, probably 50% in the last 3 years or so.
Walther LG55 £200-£250 (so many variations, DST £450+?), Diana 60/65/66 £200, 75's £250. FWB 300 £200+ depending on the model, a nice su might be closer to £300. Those would be top prices for excellent examples IMO.
Edit: Occassionally a bargain crops up. I think jimlad on here had a very nice Diana 75 @ £200 and it didn't sell. While shop prices are higher, City Air Weapons have one at £395!, and there are a number of FWB 300's at £300+.
They are still good value when you consider the quality, they should last for ever with modest maintenance. In 1985 a FWB 300su could be bought for £300 (about £750 adjusted for inflation today) which gives some perspective.