Well, if you look on any of the puppy selling sites that have pure and mixed breed you will see that puggles go for as much if not some times more than registered pugs and almost always more than a registered beagle. So that is not always the case.
I do think that people who just breed 'average' dogs are silly because if they did truly want to get more money from their puppies they should be raising the nicest puppies possible. A lady called yesterday about one of my puppies. She said she spend $5500 on her show boy from a top-big name boxer kennel. Considering the average boxer litter was 8 and her puppy wasn't even 'flashy', that top boxer breeder is doing pretty well, but it is a huge name and any one who knows anything about boxer showing would recognize it.
It a problem with both mix and purebred. I think mixes are selling for so much because they are so popular with some celebrities. Idiots see someone on tv with one and are willing to go spend an arm and a leg to get it. They don't learn anything about them first. Just run out and get them because they're the latest fad. Millers and BYB figured this out so they jack up the price as high as they can.