So your grow would be in a cabinet? I would think if you had proper ventilation in and out of your wardrobe, that would be fine just circulating the air in your garage. However, what are the temps in your garage? It would be better if you could duct everything outside, but since that isn't possible, keeping temps semi-cool in your garage would work... Opening the door during the day is even better to get fresh air in.

But yeah, I would definitely check on the AC prices if your temps in the garage are going to get really high (85+)