Anyway to find the cheapest games on the Steam Store?

http://steamdb.info/sales/

Sort by discount.

also here
lovely site.

This maybe ?

For using steams own search:

1. Set category to games
2. under advanced options set price to under 20€ or some other.
3. Click Price to sort by it.

Suprising, of 3100 games you can see nearly 3000 are under 20€ now... Real amount is less, because there is some addtional content and DLC as games...

http://www.cheapshark.com/

You could just click on the search box on the main page, press enter. Then click on price & it'll show you everything on the store from lowest to highest. You'll have to scroll past the free to play games first, but you can always filter by advanced search