Need a good Co2 controller/regulator/atmosphere control unit - how much do they run?

I need to add Co2 to my grow and have been looking around and have seen a wide variety of prices and it seems you can pay as much as you want to for one of these, so what is a good affordable CO2 regulator and atmosphere control unit? how much do they run?

Don't know for sure, but I've heard that the auto pilot is a good one. Dude said it was just like the chhc4,which is a great one.

thanks for the reply - not sure what I am going to do hmm