My favorite mono is the DSI Pro 2, but that’s neither small nor cheap.
Manther is also awesome, meets the small criterion, but aint cheap. Love the growl and it’s very hands on.
Circuit Mono Station is a lot of fun for a mono, is both small and inexpesnive, and has a very intuitive sequencer. At the risk of coming across as cheesy for plugging my own gear, I’m selling a Mono Station here: https://www.elektronauts.com/t/fs-us-circuit-circuit-mono-station-streichfett-ferrofish-b4000/190615
I would probably have it off you if I wasn’t in the UK.
I love the sound of the MonoStation but I just couldn’t get on with the sequencer. Too cryptic especially not having a screen but I think that about most of those pad based sequencers. Maybe a Ableton influenced thing? (Something else I can’t get on with!
).
My advice would be to get the BassStation2 with more control, a keybed and a sequencer. They can be had fairly cheap second hand.
Bass Station 2 is king of the affordable monos.
For someone getting started the Microbrute and Microfreak are amazing. The Microbrute is pretty straightforward, knob per function, and can be polite and dreamy or harsh and nightmarish. The Microfreak is super versatile, a dozen algorithms or so and cool capacitive keybed.
Korg Ms-20 (mini) looks intimidating but doesn’t need to be patched and has an amazing sound. When I played one of these I felt like I was really playing an instrument vs tapping a few keys and turning some knobs.
How about the 0-Coast? Definitely a lot of fun.
Then the 0-Coast and O-CTRL together!
Indeed it needs to be small and self contained so I can experiment with it on the sofa.
I have a Reface CS which is actually brilliant and ticks a lot of boxes ( #florian ), but it doesn’t have a proper sequencer so I find myself not using it.
I don’t know if the arturia minifreak would fit your budget, but it seems to tick a lot of your boxes. And it can do poly too!
I really like the look of the Minifreak, but it’s probably a bit big and a bit expensive.
The Microfreak is another potentially good choice, but my concern is that a few reviews and forum posts have said “It’s brilliant fun, BUT I never make any usable sounds with it”
I disagree.
All the sounds in this track were created on my microfreak. (btw that was before all the nice new osc types came in the subsequent updates) It is staying in my little studio for sure!
Indeed, nothing wrong with those sounds!
I think the microfreak is a brilliant little synth. You could see it as some sort of prtable micro modular thing: the possibillities of the mod matrix are very deep (eg: sending an envelope to influence its own decay to make it more snappy, sending one mod source to multiple different destinations with varying degree, mod matrix is able to modify its own mod points).
You’ll need some external effects, but this little synth itself keeps inviting to experiment on.
The minifreak does greatly expand on the possibilities of its smaller brother and also includes built in fx.
Not disagreeing but, these days, my low end Champ is the miniKORG 700FS.
I, of course, meant the sub-500 low end champ 
But I would love to check out one of those Korgs someday.
Does anyone else do that thing where they are thinking about buying a second hand Microbrute for £150 and then think
“Yeah but the microfreak is like £280 and is does loads more”
“But why would you buy a micro freak when a mini freak is £500 and has more polyphony and FX?”
“If I’m going to spend 500 on a synths it would be silly not to spend a bit more and buy a Hydrasynth”
“Well if it’s serious money were into here you might as well go the whole hog and get a polybrute”
“We all know I’m not going to happy until I get that CS80”
“Apparently it’s irresponsible to remortgage the house to buy a synth”
“Oooh look a second hand micro brute!”
Nice reminder that I always wanted to get the bass station 2.
I think I order it just now.
Anybody out there who thinks it´s no fun ?
erica bassline db-01 is the mono for endless noodling sessions. it sounds like nothing other…forget the claim “bassline”, it can do drones, scapes, textures, has a brilliant sequencer with keyboardmode, transpose sequences, arp, accent, slide, probability + more randomize features, 64 steps, pitch envelope, superb lfo-section and p-lockable cutoff. just connect it with an korg nts-1 and there you go!
I tested bass station vs Microbrute and I much preferred the raw sound of the brute and it has a great filter so I went with the brute, great mono for the price
it is terriffic fun!
The BS2 and the Microfreak are my two favorite small synths.
With bs2 the afx mode, quirky sequencer with rythm-presets and a rather flexible synth engine make it a wonderfull budget mono-synth. It can sound like a roland or like a moog and much more than those, a true chameleon. Feature-wise pretty much on par with the much more expensive moog sub(sequent)-37. Rather underrated little thing, as far as I’m concerned.