I've done it, twice. In fact, I wrote "A Spray Booth for All Seasons—and All Pocket Books" for FSM back in the NeoStyrene Epoch…
The key to building a spraybooth is simple: scrounge. Otherwise, the cost in time, effort, and materials easily exceeds the cost of a decent commercial one. Of course, if you discount your time and effort, it goes the other way.
If you can't afford the cash, by all means build your own. Your lungs, liver, brain and kidneys will all thank you. So will those of the people you live with.
The other reason to build your own is that you get exactly what you want. Just make sure that it is all that you need!