I am done building an SB4000 which is supposed to be the most current and easiest to build SSL comp clone. I haven't calibrated it yet or ran any audio through it yet, I'm waiting for some XLR-TRS cables to get here (tomorrow).
Cost:
Front panel: $30 (bought on classifieds on GroupDIY)
Case: $40
SB4000 pcbs: $60
SB4000 parts: $300 (resistors, diodes, IC's, transformer, knobs, etc)
Total cost was roughly $430. I managed to solder the whole board in one day (about 6 or 7 hours). The wiring took another 2 hours.
You can buy a new Chameleon Labs 7720 for $500, although I doubt it uses as high quality parts and I don't think the 7720 has the turbo mode and dual sidechain which is closer to the real SSL 4000 bus compressor.