It's a bit of a 'how long is a piece of string' question. An old and obscure guide is going to go for more than a recent one from the Weston-Super-Mare Aquarium, for example. That said, the prices that people ask at the Anglo- and Euro-Nerds meetings are generally very low, in comparison to the prices that some things go for on Ebay. In fact, most (all?) zoo nerds are very nice people, and very generous too, and are more likely to give stuff away than charge for it - or to charge only cost price. Swaps are a far preferable option for all, I think - and most are happy to give something in the knowledge that even if they don't get something now, they will do in the future.
It's quite interesting seeing what people go for - generally, magazines are not so valued, annual reports fairly valued, and guides very valued.
It's always a very convivial get together; I'm looking forward to it very much!