They seem to be selling via Amazon's agency sellers for between �30 - �45 secondhand, but some are selling for much less, around �14. There's one on ebay that's currently at �35.
I think what you have is just a special/deluxe edition rather than a limited edition as it's still available via Amazon (at c. �20 - RRP �30). There are similar editions for other books in the series. I don't think you'd get much more than that unless it was actually signed by JKR herself (from what you say, it's the edition where her 'signature' is gold-blocked on the cover - ISBN 0747581428).
Given that 'ordinary' editions are selling secondhand at around �3 - �5, I wouldn't imagine a charity shop price of �10 - �15 would be too much to ask, and I don't think you'd exactly be giving it away at that, either.