in australia you can get a much cheaper kit from jaycar electronics. the plans etc were released in the silicon-chip magazine. there are 3 different kinds that they have done. the original one (pretty ordinary really), a 'midi theremin' (just puts out cv i think) and the second version (much improved over the original version, with many more controls eg pitch/volume adjust, waveform etc. it is also much more linear than the earlier one). it cost me around $AU150 (the new second edition that is).
porl
ps sorry i don't have time to find links at the moment. if you are interested and can't find anything let me know and i'll look later.