I have one of these here:
http://www.lowes.com/pd_334093-74309-04530GM_0__?productId=3306338&Ntt=hamilton+beach+air+purifier&pl=1¤tURL=/pl__0__s?Ntt=hamilton+beach+air+purifier
I use it along with an Ona block. Total cost of $12 maybe. The air filter plugin cost me $7.99 at Lowes, it came with "green meadow" scented refills. It has a little carbon filter, and the scented thing. I have the Ona block almost completely closed. I can't smell anything but the "green meadow", and a little of the Ona. My grow area is pretty small, 2 x 2, with 5 small plants, all 16" and under.
Once the filter needs replacing, I will modify the thing and see what I can do to it, those replacement filters cost more than the unit. Activated charcoal is pretty cheap at the pet store.
I almost bought one of those bigger fan + carbon filters, for like $150. But for a 2 x 2 area, that is a little much. What I have is working great. Doubt it'd work for a larger grow though.