Depends on your definition of 'useable' but you'd certainly get an early VA board 2nd hand for $300-400 like the Yamaha CS1X. For monophonic new you have optio

Depends on your definition of 'useable' but you'd certainly get an early VA board 2nd hand for $300-400 like the Yamaha CS1X. For monophonic new you have options like the Korg Minilogue Behringer MS-1 IK Multimedia UNO Synth Pro Novation Bass Station II Yamaha Reface series