Here's my opinion. It is 100% worth the extra cost. Why? Not because you're guaranteed a "disease free" tank, but because these fish are free of the major killers in a tank. If you can get a tank that doesn't have ich or velvet, the rest are neglible. Ich and velvet are the destroyers of a tank when they get out of hand. Bacterial infections? It's normal for a fish (just like a person) to get infections.
Feed your fish a whole, balanced diet and they'll weather through any storm. I have a tank that has ich in it. I haven't had a loss in that tank for over a year, and that was after I put in a fish that came from a disease ridden tank at my LFS. None of my other fish caught it's funk. I've watched the fish in that tank get ich and bacterial infections several times over the last few years. It continues to be less frequently. I've had the same 5 fish in the 50 gallon for 3 years (minus the 1 loss).
This all being said, I'd never put any of these fish in one of my other tanks without putting them through copper treatments, even though all of my fish have incredible immune systems now. I believe that even if I did introduce ich into one of my "disease free" tanks, not one would perish.
Look at BRS's most recent interview with aquabiomics. They've tested Ryan's 360 gallon tank and it still has signs of uronema
This is after he purchased them through Marine Collectors.
It's time this hobby starts looking at fish disease a little differently. There's no reason you should accept a half dead fish, but
@Paul B has an awesome take on fish disease and it has completely changed my opinion on the topic.