should be doable for a pretty darn decent mini system for under $600
My controller was 450 (sentinel cppm-4)
Regulator- 150
Tanks- used for $50 for each 20 pounder...I basically stole those tanks from that guy AND they are aluminum with plastic handles.
If you are doing a sealed setup, you gotta watch out for which CO2 monitor you purchase. Many of the cheaper ones are designed for non-sealed rooms, so when the lights are on and your temps get high and your extraction fan kicks on, it disables the co2 while the fan is running because it assumes it is exhausting the room and not just a sealed light system. I bought the cppm4 because it works completely independant from my fan controller, so when the lights are on and it's spraying co2 and the fan kicks on, the co2 keeps on a going. I use a day/night controller for my fans with a photocell so I can adjust my day temps and night temps seperately without affecting the co2.
And remember, CO2 is heavier than the air around us, so you need to place the tubing either right through the middle of your crop, or above it. Don't just let it spray out the bottom of the regulator or alot of your co2 is just going to hit the ground and stay there.