It's been a while since I last researched it but the least expensive unit I've saw at the time that was reportedly accurate is Dylos DC1100 monitors for ~$200-$300, depending on the sensitivity and options desired. However, I never got around to purchasing one so I can't vouch for it personally.
I've got a Dylos DC1100 Pro and it seems to work great, however I don't have a solid way of verifying the accuracy of its measurements. Supposedly it's one of the most accurate devices available at that price.
Empirically it works; running a HEPA filter in a closed room for 15 minutes will eliminate almost all particle readings. Cooking bacon in the kitchen will make the meter read 100x higher than normal, etc. I'd recommend buying one if you're interested in monitoring air quality, if anything it will be useful in giving relative quality measurements.
I second this question- please post some links if anyone knows of an effective way to detect and do long-term monitoring of these in-home or at a workplace.
