I used to have a frostwave resinator, good filter but i sold it as it was only mono..

Ive used the electrix filterfactoy & filterqueens a lot and they are ok, but not quite as big sounding as a sherman, akai mfc42 or frostwave.. but still a phuck of an improvment over any vst filters. Actually they do go pretty cheap though so for the money they are stilll ok.

Id check out the mfc42 - you can get em around 150 second hand and they are very phat sounding, thats what i use now.. although id also like to get a mutronix mutator to join it for extra filth - but they are quite expensive in comparison. The akai is the best for the money for sure.

The electric Eq Killer and mo-fx are both well worth checking out..

For more basic stuff ive seen TC M-one's go for around 150 on there which is a bloody good sound for that kind of money