Had I known this 6-12 months ago, I would have ordered all my fish through
@Legonch Emerald Reef Supply. So if a fish normally goes for $80, it's worth double that if someone QT's it correctly for 30 days. Here's why. First, there is a high mortality rate of fish in the first 30 days. Most business that deal with fish are what I call "burn and turn" as it's received on Thursday, quick QT/ Dip, and it's in a tank to sell for the Friday crowds. Fish going from ocean to your tank go through a lot, it's stressful, and it happens... fish die. When I order fish online, I often order 20-30% more knowing some will die. Yes, you'll get credit, but it happens... even with Diver's Den who does a good job at QT.
Second, you have to buy all the chemicals/treatments - like what Hal3134 mentions. That's a cost.
Third, there is much debate on how long to QT - from 3-6 weeks. It's not hard, but a pain. Think of feeding two tanks, and depending on setup, taking care of two tanks. It's not like you're getting one batch of fish, most likely x2-4 batches so several months... believe me, I am dealing with this now. How much is your time worth?
Forth, in 4 weeks, a fish is settled in and eating. In addition to stress/disease, this is the second most important reason for QT, especially when adding fish in a tank that already has fish. They have to "hold its own", compete for food and not get bullied. All these factors is a circle - fish that are stressed, are more susceptible to disease. Diseased or stressed fish don't eat well, fish that don't eat are even more stressed, and more susceptible to disease/ All this leads to a loss. It's a hard cycle to break once it starts.
I lost about 25 fish due to not QTing correctly and getting velvet. One mistake here in this area could be costly, and painful to watch.
All reefers should have a permanent QT setup. I would get fish from Lagonch, and keep them in a tank for one week just to ensure they are eating your food, and use to your water. That is sooo much easier than doing the whole QT process.