What pray tell is a "Smoke Test"? I think it's for checking if you've got an air leak, but am just guessing. Cost?

 

Smoke tests are used to find various kinds of leaks - vacuum leaks and fuel system evap leaks are common culprits.

Depends on what's being tested, what symptoms you're trying to diagnose, how long that takes, and the shop you take it to - most shops have a smoke machine.