If you're on a budget, you can always scope what's around your local pawn stores. Try to avoid any Casio keyboards you see, because you really can't edit the sounds, you have no filters, A/S/D/R control, or much you can do to make original sounds.
You can always also check out guitarcenter.com or roguemusic.com and they will have something in your budget. If you have the money to go for something new, go ahead, but if you're unsure what to get, think of the sounds you're after. Are they analogue with fat bass, warm pads, or 8bit chip sounds? Get a analogue or virtual analogue synth. If you need a plethora of sounds, that sound like acoustic instruments(piano, guitar, violin, fret less bass, etc), you would want a digital synth.
Do some homework at
www.vintagesynth.com or sonicstate.com, and try to see the pros and cons of what's out there. You could even just get a USB keyboard controller, some software, and a decent audio interface and get sounds this way before you spend serious money down the road. Kind of depends on what you see yourself doing. It's up to you.
