Not sure off the top of my head, but you could use eBay as a rough guide - these are some auctions I found (only auctions with bids really count - just because a seller has a buy it now price of �200, doesn't mean it's always worth it!!)
These are the current values of the phones I found...
Nokia 6300 - �119, 7 bids
SE K810i - �180, 6 bids
SE W880i (Still sealed) �150, 1 bid
SE W800i - �71.50, 2 bids
Samsung U600 (Still sealed) - �170, 7 bids
Samsung D600 - �145, 1 bid
Motorola KRZR K1 (Pink) - �108, 10 bids
Most had a day or two of auction left, so they could all increase!
Hope this helps :)