The cheapest one I've seen is the Way Huge Ringworm for ~$130 (on the 'bay, ostensibly new) then the EHX Frequency Analyzer for ~$150. I wonder if the price is due to demand or production cost. Maybe Behringer will make one for <$39.99 that may or may not catch fire during use.
DOD FX-13 Gonkulator Modulator. I don't think they make them anymore, but they pop ut used every now and then. I got mine used for €50,-. Mike Einziger from Icubus uses the pedal in "Glass" (and several other songs) if you wan't to know what it can sound like.
Got my Frequency Analyzer in the mail today. Very cool and just so much better than the crappy ring mod patches featured on most multi-fx units.
Of course, it's very easy to get a crazy, fucked-up sound out of it, but it's also fun (and surprisingly easy) to tune it to a desired key and play around with contrasting consonant and dissonant notes.