I got my Milwakee Co2 Regulator from J&L for $90 (
Milwaukee ma957 Co2 Regulator). comes with a regulator, needle valve, counter and a solenoid. Not bad for what you get.... (I don't know how long they last though... especially the solenoid part)
vdub, I looked at the Tankspiration.com website as well, and the cost adds up to $120 ish.
I set up mine for:
$60 - Annual lease of 20lb tank
$30 - gas for 20lb tank
$90 - regulator
= $180
Out of this I get a 20lb tank, the tank is on lease so 100% guarantee that the tank is hydrotested (since it is from praxair), and I'll get a 'new' one each time I order the gas.
The diffusion part can be done cheap (many methods online).
I looked for 3 months for a cheaper way to do it (called almost all of the local retail stores, plus non fish related supply stores). Went to Canadian tire, called lab companies, etc. There wasn't really a lot of ways to do it. In the end the regulator is the only thing you can get for cheap (Co2 and cylinder is kind of set), and you DO NOT want to mess with that! Putting a hole in the side of the house because you got a cheap defective one is not a good idea. I personally would not feel comfortable working with a regulator that I don't know where it came from.