Is that your absolute $$ maximum ?
'Cause I'm afraid you will not find any synth below that price. (*)
The K-station from Novation is a little more (about $ 700), but then you still don't have a sequenser, and it's also not a "remixing" synth. Or do you already own a sequenser (like Cubase or other recording software) ? And do you have already a MIDI keyboard ?
It would be easier to help if you gave any more information.
(*) You can always go check eBay for 2nd hand gear; there are very often real bargains there;
a Roland Juno 106 is a decent synth, but without sequenser / arpeggiator / fx and "remixing" capacities.
The best all-in-one are:
* Reason (only software but with sequenser, fx, sampler, synth, remixing capities); that's less than $500
* Akai MPC2000 / MPC3000: but then you have to save much more money, sorry... ; $2000-3000
Peace !!