Hmmm not much now I'm afraid. If it's the PCI-E version I'd say around £15-20. There are newer generation cards like the 9500GT and HD 4650 which comfortably outperform it, have double the memory and can be had for just over £35 new, so it's lost a lot of value.
If it's the AGP version then even though it's not as good as the PCI-E version for performance, it will still command more of a premium due to the rarity of decent AGP cards. It's better than the more common 7600GS which I still see going for over £25-30, So I'd guess at about £30-35.