Budget synth for basslines?

The Electribe's are not exactly full-blown synthesizers though. So keep that in mind!

I recommend the Volca Beats, as suggested before. Best bang for the buck at $150 for real analog.

The Minibrute is also a great deal, now that the price dropped from $499 to $399.

I would rather suggest you get a super-cheap midi-keyboard and sync it to the s950 which actually is a synth itself. It has a really thick sound to - perfect for bass.
Just sample a few bass sounds into it and save to a "Bassline" floppy and you're good to go.
Even cheaper just use 16 levels with the s950.

https://www.youtube.com/watch?v=BFkReUYIbM0

this dude uses a s900 with his mpc2kxl if you want sound examples.

Novation MiniNova (18 voices) comes with Virtual Analog Oscillators, Wavetable oscillators. I don't own this, but for someone starting out you get the analog emulation plus newer oscillators to really build off of that when you get better at synth sound design.

Dave Smith Mopho Keyboard and Novation Bass Station II (both monophonic) is also great to get because most of the parameters for synthesis are there. Once you understand these fundamentals of synthesis controls better you can go into something like a MicroKorg or MiniNova and program patches quicker. Both of these are well built so buying a used one to save cash is alright.

Oberheim Matrix 1000. These can only be found used and for a good price. It's a module, but if you combine your ipad with usb to midi you can program it through your ipad and this will give you 6 voices of analog tweaking.