We use a portable A/C. It is 8000 BTU, so it is a small unit and is rated to cool a 10x10x10 room 20-25 degrees or something, so in reality its good for lowering it about 10-15 degrees... Because we supplement with CO2, this allows us to get rid of all ventillation. It doesnt neccessarily lower cost because your running an A/C and dehumidifier and CO2 but the results are well worth it because it allows you to finitely adjust the environment your plants are in. Because we use CO2, temps of 80-90 degrees are actually not bad for us, so we do not need a terribly large A/C. The dehumidifier puts out considerable heat sometimes, so the A/C is pretty much there to counter that.

The portable A/C unit does produce considerable heat, which is easily vented outside or into an attic or for us (just into the house for warmth, granted the CO2 is off). Now if I had a choice to have a window mounted A/C...well of course I would do it, but its absolutely out of the question for us. Marijuana does like high temps, especially if its a tropical strain, but there needs to be ample air circulation and supplementation with CO2, etc. When the lights go off, we run the A/C to cool the room for the night otherwise it can stay pretty warm in there. But pretty much for us, the water chiller is the #1 priority because the plants can handle plenty of high temps...its the roots that cannot.

PS. Remember, the A/C cannot be run while CO2 is present as it will just vent it out of the room. Therefore you must use a schedule or youll be wasting CO2. We cool a few times a day, then bring the temps back up. This is a big change from my old CO2 regmine. Oh and we are running 4 aircooled 750 digital lights with a recirc hydro system. So because we are not using the A/C to cool the water, just the plants, it doesnt need to be powerful. The unit was $150 used.