firstly, sack keys off mate, you're gonna get something insanely shitty for that price.
secondly, buy second hand, if you're gonna spend fuck all it'll most likely keep the resale value.
thirdly, save more cash, seen arturia microbrutes going for about 160-180 second hand, well well worth that little bit extra.
if you wanna ignore that then here's some stuff
korg volca keys or bass.
second mutable instruments shruthi-1 - sounds awesome, digital synth with analogue filters - i want one badly!
waldorf rocket, also want one badly!
dave smith mopho desktop - absolutely bitch to programme and probs not worth your time
Some stuff to maybe look at around that price range would be a microkorg (used) or maybe an old yamaha an200 (no keys but nice lil synth) Maybe an Ion. Another keyless cheap synth would be a Korg electibe ea I think? For your price range I would think groove box with a controller might be your best bet. 100? is not much for a piece of gear. Double that and you have some play. Also depending on what kind of synth you are looking for. Analogish? Fmish? Subtractive? Good luck
I could only find the Korg Volca for about that price range brand new. If you are willing to pay a bit more, the Novation Mininova, Alesis Micron or Arturia Microbrute (+ other korgs) would be worth looking at.
Don't buy a hardware synth for 100(whatever).
Good synths start around 500€ and nowadays i think they are not essential for tight productions anymore, because the sound of vsts is pretty close to all the digital hardware stuff.
Great thing about hardware is that you can touch knobs, press buttons, play the keys, bend the pitchbend... This makes me, personally, feeling the sound more then just tweaking some stuff with shortcuts or my mouse.
Or buy an analog synth, the sound is usually fatter, but you will have to pay a huge amount of money to get something valueable.
For 100€ you could go for some great little midi keyboards that you can always use in a good way for your production. E.g.: http://cme-xkey.com/ (99$)
Already have an akai mpk49 or whatever it is, going to sell it and try and buy a half decent synth, not bothered about keys anymore, Il probably spend around 250 as im selling my keyboard.. May get a Roland ? any suggestions?