Well, the cheapest way is to build one yourself. The supplies are cheap and you can build it to your specific spec's provided you have the properfan/blower for the spray area. There's many plans available on many websites, all you have to do is google. The best type of paint booth to buy or build is one that uses a furnace blower, unlike the paint booth in your link. Keepsflammable fumes out of the fan motor since it uses a squirrel cage.

If you don't want to build one yourself, then you can purchase one from this site. It's a quality build, uses powerful furnace blowers and made of sheet metal. I purchased one from Bob years ago, but eventually my work called for a bigger booth, so I sold it.

http://www.pacepaintbooths.com/pace/