I have purchased many different types of air purifiers over the past few years (for my work, as a means of removing dust particles in the air since we work on electronic equip) and the best brand I have used is IDYLIS. They are sold at lowes, if you go to lowes.com and type in idylis you will find them all. They have several models the cheapest is arround 90 bucks, it says it is good for rooms up to 155 sq. feet. that was the first IDYLIS air purifier I purchased. It worked so good that I decided to purchase a second unit (it is for rooms up to 310 sq. feet). That one cost arround $280 dollars. That unit actually has a meter built in that measures the quality of the air. I purchased a third unit on ebay- the same model as the second unit (310 sq feet) for arround $75 dollars so that was a steal.
One thing about the square feet is they are just suggested by the manufacturer, if your room is larger it will still work.
To save money you dont have to throw the hepa filter in the trash every 6 months like it says, I actually dust the filter out with my air compressor, if you spray air from the back side you can blow the dust out of the filter (put the air nozzle against the back of the filter to spray air, dont spray from the front of the filter or you will be pushing the dust particles into the filter which will clog it). You can do the same with the carbon filter if you are on a tight budget. I would atleast replace the hepa filter yearly.
Before I had these filters if I dusted a shelf off in my office and checked the shelf a day or two later I would see a layer of dust built up. With the filters running I hardly see ANY dust on the shelf even after a week or two.