Here's how to build yourself an analog mono synth with £60 and an old cigar box:
Visit Ray Wilson at Music from Outer Space
Send him $30, and he'll send you a printed circuit board for a tiny battery powered monophonic synth. Details
Spend some time at a fantastically geeky site like All Electronics or Maplin buying the other components. The whole thing should cost $30-$60. Apparently pots are the expensive bit. Don't get distracted
Find a case. This is the fun bit, judging from the gallery
Assemble
Play!