I think the cheapest is the Behringer Hell Babe.
I had one, for a while, but couldn't get on with the on/off switch, which is a shutter gate at the "full-up" pedal position (i.e at the heel end). This means that, when you switch it on, by moving out of the shutter gate, you suddenly go into full "bass" wah, rather than full "treble", as with most pedals. "full treble" is closest to where you are, when the wah is switched off.
It also means that, if you get too enthusiastic, and rock the pedal too far back, you switch it off.
Other than that, it is a nice wah. I would have kept mine, had it had a decent switch.