Don't buy the cheap ones, they suck (and not the air for filtration). I started out with a cheap one for like $60 and didn't like it and still have it but pretty much never use it. I have one that is really powerful but quite noisy. This one I mostly use during the day when I'm not home. Another one I have is quiet enough in the low setting that I use it 24/7. Even though it's on low, the length of time it is running helps.
This is the one I have and I really like it. It's quiet and effective. It can be put on swing mode where it spins side to side or lock in one direction. When I take the birds outside I turn it up to full blast but keep it on low 24/7. Don't waste your money on a cheap one that you won't like to use anyway. Perhaps save up for a bit and get this for the long run. I ended up wasting money on the cheap one because I had to replace it with a good one anyway. Shop around though, you may be able to find a better price on the same unit elsewhere.