after extensive research I believe this is the best site I found, too bad 90% of the purifiers listed on it I couldn't find anywhere in Europe, but if you're in the States then you're lucky:
http://www.allergybuyersclub.com/compare-air.html
I ended up buying one for ~ $500.00 since I have hay fever/alergy but actually that didn't help me much, going to the doctor did ;)
It's quite simple - if you live 24/7 indoors then air purifier is great, otherwise it's not much of a help.
So if it's alergy you want to buy it for then don't, see a specialist, the pills they sell now are much better than the ones they were selling even a few eyars ago