It's worth what ever the market will bear for a gun like that. Google has a few buy and sell ads for this gun from the UK and the US. But they are all from 2007-2009.
http://www.airsoftforum.com/board/to...-hk51-limited/
http://www.neasg.org/showthread.php?...package-*RARE*
Value depends on the condition as well and what the gun comes with and if its sought after from someone.
If you get Av'd on here you could try to sell it on here.