are you looking for a room style one? or one to go on your forced air furnace?
I have had Aprilaire's Space Guard 2200 filter which gets changed once to twice a year, locally the replacement media is about $25.00
I have also owned a Honeywell electronic air cleaner, while it did clean the air well, i didnt care to have to wash it every one to three months, and when it gets too full of dirt, it starts to let go of it back into the ducts
I currently have Carriers top of the line filter, which off the the record they claim it will kill anterax. This filter is very expensive, and to date i cant say i see a difference between this and the Space Guard unit.
We all have allergies in my house and the pollen from living in the country can be brutal at times.
We have a 3M air purifier with 3M Hepafiltration filter, was cheap and the filters are pretty reasonable at $20 and need replacement about every 6 months. We run ours 24/7.
I Like the Healthstart ION-70, it does have on going costs of filter replacements but the germicidel light and the hepa filter are worth it.
if those were my choices and it was my money thats what i would buy.
we don't have forced air. we have a cat, so sometimes my allergies flare up. We bought two ionic breezes and a venta airwasher, and it has made a huge difference. We put these things where we spend the most time (i.e. living room, bedroom) and it has been great!
I recently adopted 2 cockatiels from my girlfriend, and they give off a lot of dust, small feathers, etc. and I invested in a $50 Honeywell HHT-011 portable air purifier. It seems to be working very well so far. The foam pre-filter is already coated in small feathers, dust, etc. although the HEPA filter itself still looks perfectly clean. Has a built-in Ionizer feature and 3 different speed settings. I like it so far, but I've heard it has a LOT of energy consumption. It says it's only for small rooms, though. The filter is permanent, which is nice.