Some great suggestions here, most of which fail the 'cheap' requirement by some distance.
I'd go with Bigwan's suggestion of the MS60B or B1on. There are a couple of OK synth patches, which can be surrounded in the chain with the other bits and pieces to make them work better ie octaver, EQ to help with tracking, chorus and delay to fatten up the sound a bit. Or you can chain together the various fuzz, envelope filter and octave effects.
Another option might be the Ibanez SB7 in the Effects For Sale section.
Or do the sensible think and buy a cheap keyboard synth.