for that price range, you will find something like a used microkorg or maybe an alesis (i forgot the model...), both of these have vocoders, which is cool!
another option, which is a lot of BANG for the buck, is the AU/VST plug scene. if you have a computer (could be a used ibook , i got one for around $100usd). a program like garageband or cubase...
then do a google search for KVR AUDIO. there are some awesome FREE plugs like DA HORNET (wasp synth emulator), and TALEC7RO (a 3 oscillator synth, pretty fat!). there are deals if you look hard...i found a novation bass station plug that works with my PPC mac.
find a midi controller like m-audio ozone, oxygen, etc. the korg nano series rocks too! sometimes you can find this stuff CHEAP used! i got my ozone for around $40 used, and my nano kontrol for around the same price.
if you are frugel, you can do this whole setup for around or under $300usd.
if you want to spend a little more and get something NICE, the dave smith mofo is a great REAL analog synth for the money! mine was around $600 used, they go for around $800 new. but they have a lowpass vcf, 4 lfos, dual oscillators, sub-bass generator, sequencer, arpeggiator...that is a LOT of synth!