Check out the blog

http://particlecounting.tumblr.com/

The writer has tested a lot of combinations of fans and filters (Simple, HEPA, Activated Charcoal). It's astonishingly easy to build a combination that costs way less than a "professional" Air filter.

The writer has put his findings into a "social startup" that builts these simple, but very effective airfilters in some developing countries:

https://smartairfilters.com/cn/en/product-category/diy-air-p...