For real or for fake?
Controller ~$400
Tank/Reg/Valve ~$150-$200
scrapping the controller, you might as well guess at the temp, nutes, and pH also.
Now on the cheap, vinegar/baking soda or sugar/yeast methods produce CO. (BornToKillSpam has a *killer* post on the yeast method) but of course its all uncontrolled. TONS of posts about it. do a search.