You get what you pay for with air purifiers. The expensive ones cover more space, and are generally better. The "one room" ones only cover one room, and sometimes not even that.
My boyfriend and I have been lucky to find several great air purifiers for cheap at thrift stores. Some of them cost around $600 new, but second hand they can be much more affordable. I paid $100 for my LivingAir at a "Fix-It-Shop", which was great when I lived with cats, and now that I don't, it's still great. My boyfriend found another couple at thrift stores for around $20. LivingAirs, and similar ones (a lot of them look exactly the same but have different names, haha), are great.
Whatever you decide to get, make sure you look at how much area it covers. If you're worried about covering your whole house, as opposed to just a room or two, a "fancier" one would probably be better. Besides, the good ones seem to last forever, so it'll definitely pay off.
I've had mine off for a week or so because I've had the window open, and I washed the filter, and haven't put it back in yet. I just realized that I'm sneezing a lot more now than I was before! Even though I only have a lint filter in mine (haven't felt like investing in HEPA filters, although it does take them), it still makes a HUGE difference!