I built one of these for under $100.00 from parts at Home Depot / Lowes.

eBay: 4" X 12" ACTIVATED CARBON AIR FILTER/SCRUBBER (item 180090500486 end time Mar-05-07 21:41:07 PST)

I added a 440 in-line Eco Plus fan ($140.00) to draw air and scrub the room. This sucker works the best out of everything I have tried for stealth.

The activated carbon was bought on E-bay for $45.00 and came in a 5 gallon pail and I used half of it to fill the scrubber and will last me three years or so.