The larger (120 mm ~ 4.5 inch) fans appear to be about five times more expensive, or more, than the 80 mm varieties, and they move only about twice as much air. So if you want to economize it might be good to get one of the bulk orders of smaller fans and use more of them. You can get ten 80 mm fans for less than $20.00 shipped on ebay.
I haven't finished any of my projects, so I can't testify to the success of my plan, but I think I'm going to use two of the small fans on each of the MH fixtures in my canopy, to keep them cool, and then move air both in and out of the canopy with two or more as well. I've cannibalized several 12 V DC transformers from light fixtures and bought one on ebay to support this (depending on the rating of the transformer it may only be able to support one, two, or five of the fans, which each pull about 110 mA).
It may be days or weeks before I have time to implement this (to replace the larger fans I'm running in the meantime).
FWIW,
Brad