I paid well over a hundred quid for mine a few years ago - and that was from a personal friend who cut me a good deal.
Everything is worth what you can get - but you should be able to get a decent price for it. Not one to throw randomly in the Oxfam box unless you feel REALLY generous. (Things in those get all wet and horrid anyway - such a waste ! Take it in in person if you want them to do well - and TELL them it is worth something. In fact - use their specialised bookshop for something like that...)