Modulars are great and all, but if you can't afford a $2,000 > system, why not modify an existing synth? There are lots of cheapo circuit bending project type synths, but "real" vintage synths can be modified to accept inputs and outputs.
For instance, I modified my MC-202, so that it's more like an MS-20. It has: CV VCF IN (filter control), audio -> VCF IN, PULSE audio OUT, TRI audio OUT with level (original MC-202 has no TRI wave), SUB OSC OUT, MOD In, and upgraded CV/GATE INs (so that they bypass the CPU). You can patch anything to anything and it's a really cheap and flexible way to get into modulars.
Go here for more ideas:
http://www.cykong.com/Synths/synths.htm[d]