Back before Christmas, these went down to $120 on Walmart.com:
Buy Coway Air Purifier Airmega 200M True HEPA with 361 sq. ft. Coverage in Black at Walmart.com
www.walmart.com
I picked up 2 of them to run on opposite ends of my house. There's an air quality light that changes from Blue to Purple to Red depending on air quality. It's a 4 stage filter with ionizer and we can tell a difference when we run them. When the air quality detection drops, the fan increases speed automatically. We notice when we're cooking in the house and browning meat or vegetables in oil that it really works to clean the air up. We also notice that they sometimes come on and we don't know why.....but we know it's detecting dust or something in the air that we can't see.
Filters for these can be picked up for $25-30 and reviews are pretty good. There's another similar model (1512h or something like that..) for a similar price. They're about the same size. Don't bother with the smaller units. They just don't move enough air to really be effective if you ask me. For these, they suggest cleaning the washable pre-filter every 2 weeks. There's a charcoal filter behind that and then the hepa filter. Inside the unit is an air ionizer....(those are the 4 stages)