At our local NCT sale there tend to be loads in great condition for a fraction of retail price.
If you want one that folds down easily this style is good http://www.elc.co.uk/Tiny-Love-Gymini-Animal-Concert-Play-Gym/132343,default,pd.html
The arch bit goes flat and folds in so you just have two long pieces side by side, and the mat isn't over-padded so just rolls or folds up (and washes/dries easily and quickly too, for the inevitable vomit moments)
I got the same one as pootlebug (I think, the link didn't work) and DS loved it. I got it second hand from eBay and it's still good enough to pass on after several washes.
I would say look for something cheap but not something where the toys are too high for a baby to reach. My mum made that mistake and it was pointless for a while as DS just frustrated that he couldn't hit the toys. eBay is definitely good for bargains!