There are many different types and calibers of personal air purifiers.  These go from low quality to very high quality, and as with any product, the price corresponds pretty well to the quality of the product.  Brands that have made names for themselves, and have proven beyond any doubt that their products work and operate effectively, have earned the right to charge higher prices for what they sell.  If you want to take the lower priced route, that is fine, but understand that you get what you pay for.

For some people, the cheap air purification systems out there might work for them.  If you do not have any breathing problems and just want to improve your air slightly, a cheap air filter might be enough for you.  Not everybody needs the best of everything, and indeed nobody can have the best of everything; you need to make decisions and sacrifices, and if you do not need an extravagant air filter, then there is no point in springing for one.

The next step up would be medium-level air filters.  You would definitely want one of these if you have breathing problems, and for some people they might be able to fix those problems completely.  These kinds of machines can take out most of the major inhibiting particles in the air, such as dust, dander, pollen, and smoke.  These air filters cost a few hundred dollars, and they can last a long time, while retaining operating efficiency all the way through.

The final option would be the highest quality air filters.  These filters can take out particles much smaller than lesser filters.  They use techniques such as ultraviolet germicidal irradiation and photocatalytic oxidation (interesting names, right?)  These methods can remove far more contaminants from the air than simple filters can, and are great for people with serious breathing difficulties.  If you have a lot of money, there is no reason you should not get yourself the best air filter out there.

To determine what you need, you have to give yourself a self-assessment.  You do not want to pay more money than you need to, but you want something that will fit your needs as well.  You may be the kind of person who really needs an air filter for their health, and in that case, the price is not as important as the relief that you will feel.  For the average person, a mid-level air filter would probably work out fine.