5lb. CO2 tank filled - $60
Milwaukee regulator with CO2 tubing- $69
Dennerle CO2 check valve - $18
DIY inline reactor - $10
Bigpow, the cheapest you'll get the Milwaukee is from glass-gardens, for $65 (discount to PT members). You'll also need a good checkvalve for the Milwaukee since they don't have one, and those can be around $18 (Dennerle from marinedepot.com). A JBJ from aquatic store is $89 (free shipping) and already has a checkvalve. So looking about the same for either or. CO2 tanks vary locally.
There is an article written somewhere on this forum to do it all for under $100. By this you won't be using a Milwaukee or JBJ, but instead a regular CO2 regulator and a clippard needle valve. For me, I wasn't saving much money that way - I could get the regulator for $40, and the clippard with adapters for $20 = $60 total. All depends on who your clippard dealer is too. I just payed $9 more for the Milwaukee to save the hassle.