I've gone through enough money, time and heartache over disease problems though the years that I QT everything that goes into the tank, be it fish, coral or rock, for 90 days.
Sure, setting up a second QT tank for corals may cost a couple hundred dollars or so...but what's that really, compared to losing all your SPS to a few nudibranch eggs you didn't notice? Any other parasite, for that matter...the cost of treating one outbreak in the display, from medication to time and energy and the inevitable livestock losses is almost certain to far exceed the cost of that QT setup.
Eventually, if they stay in the hobby long enough, I think most hobbyists reach that same conclusion. It's worth the cost and then some.
As to costs, it really does not have to be set up with the equipment of a full reef. Corals will generally greatly benefit from a period of reduced illumination when you get them, so you don't need to spend a ton of money on lighting a QT tank (choosing a shallow QT tank will help greatly with this as well). A simple powerhead, heater and a seeded sponge filter will do the rest. I don't use sand or live rock in my QT tanks (although I do put some sand in a tupperware tub for burying wrasses or pistol shrimp).
