Well, for that price range, the best "dust sensor" that I have seen data on is the Grove - Dust sensor.
http://www.seeedstudio.com/wiki/Grove_-_Dust_sensor
It runs the closest to those cheapy (~$290) laser particle counters when compared side by side:
No resolution other than a relative "dustiness" read out on analog. The only reason this one rises above the rest are the two pots. I like knobs 
You can at least calibrate it to something.
Hope that helps.