If the duct run isn't too long, I think this one will do it (edit: for 4" tubes). Rated 40CFM. We're only cooling one light, not venting an entire grow op. Presuming you already have a main exhaust fan.

For 6" something like this will do, but we're back to $57 and 250CFM (also presuming short duct)

If you bring in air from just outside the op and dump it just outside as well, it won't need any scent treatment. You'll only need to use care not to allow warmed air to get sucked right back into the op.