I bought a 3 pack of these filters for my forced air furnace. The Flanders company makes these and they appear to be very well made. This company has been in this business for over 50 years. I am going by memory on the price which if I remember right was about 20 bucks for a 3 pack.
I put in the first one at the start of last winter. We have had no problems with it and it cleaned the air just fine. No odd smells etc. over the course of the last heating season. They claim it filters under ordinary conditions for about 90 days. These are not washable filters. You throw the old one away and install a new one. You put a new one in at the start of each heating season.
They certainly are cheap enough and the company makes good quality filters. They sell different filters with varying levels of effectiveness. The cheapest appears to be the flat panel spun glass, then the one I am reviewing the standard, then comes the plus and after that the elite.