Hmmmm, bunch of options here.
Kind of depends on where you want to go.
A good first step would probably be something with a real keyboard. You have to get one sooner or later anyway to control all the toys that will pile up. If you have keys (or get them cheap), there are a bunch of fun little Desktop modules. Maybe a Groovebox would be right for you? Don´t discard the option of going with a controller keyboard and a computer.
Ill just throw out some random options:
Options for new stuff with keys:
-Novation Ultranova: reasonably simple and nice sounding. If you like supersaws, that would be the weapon of choice.
-Roland Gaia: Less power and sound, but a very nice interface.
-Akai Miniak / Alesis Micron: Both are the same synth. You might get it dirt cheap, the interface is terrible, the sound is quite nice.
-Blofeld Keyboard: Upper end of your budget, seriously powerful synth, a bit tricky to get a nice/friendly sound from.
Used with keyboard:
- I´d say an old Casio CZ. Cheap and interesting sound.
- An1x: The interface is a bit ugly, but not bad, keybed is nice, sounds are very nice.
- Some Yamaha FM Synth. Not really fun/friendly, but a lot of flexibility.
- A used workstation (the good ones could still be a bit expensive).
- An Ion might be in your price range.
The expander/desktop modules:
-Blofeld: Fun, but a bit flimsy. Great for nasty sounds, Not so great for warm/full sound.
- Evolver: Not quite standard analog sound, fun sequencers, great for bleeps, chiptunes, drones, etc.
- The MFB ...zwerg stuff (e.g.: Microzwerg, Schlagzwerg): Cheap (at least in Europe) fun analog stuff.
- The Electribe from Korg is more or less the standard box. Not the most spectacular engines, but generally considered fun.
- If you get lucky you might find a used Spectralis within your budget. Not sure if I really would recommend one to you (very steep learning curve), but it´s basically the ultimate groovebox.
I 2nd the Minibrute.
If you like the monotron you might want to check out the monotribe. Get the midi in out from amazing machines and for a little under $300 you have a nice (but very basic) analog mono synth.
probably something polyphonic and possibly digital to get sounds that the mopho misses. My Korg R3 seems to be able to get every sound that i want to get out of it, though its a bit of a love/hate relationship with its sound character. maybe thats just me
I like to think Korg released the 'trons to test the waters of analog synthesizers and that they have more interesting analog monosynths and maybe even polyphonic ones in development. One can always dream.
For now, the MFB Nanozwerg or DSI MoPho would be a nice step up for you.