I'd go along with what everyone else has said (I used to have a DX21 myself & got some decent sounds out of it), & at that price you can't go wrong. However, have a poke around & see if you can find a DX11 for a decent price (I paid £60 for my first one), it's more recent, better buttons (think DX7II), multitimbral, and has velocity & aftertouch. You might not find one for $50 though. Alternatively, get the 21 & look out for an 11 later, they're both 4-operator, so patches are pretty much interchangeable.
Casio PX-320, Odyssey Mk.1, Multiman-S, Andromeda A6, DW-8000, JD-800, Yamaha DX11, AN1x, & SK30, E&MM/Maplin Spectrum, Akai S3000XL, E-Mu Proformance 1, Esi-32, & Classic Keys, Alesis D4, Rickenbacker 4001.