How much money does a satin silky (from a good breeder!) cost?
Are there some sort of tests the person should be doing on their breeding stock?

Tests? What are these tests you speak of? Seriously, there is no offa for guinea pigs. There are no shots, there are no tests, there is no dna profiling. They might give you a worming record, and a sample of whatever they feed.

You pretty much look at the critter in front of you. Does it have all of his front teeth, all of it's toes, is it the right sex, does the fur seem in good shape, anything dripping out of any end that looks abnormal? Are the eyes goop free? Decent body condition? Do the other pigs in the carrier all look healthy and clean?

There may or may not be a pedigree. The pedigree should have colors, a ear tag number, a weight (in oz) but unlike dogs registered animals are the exception, not the rule. Also it's not unusual for different breeds to be on the same pedigree. A silkie, and a silkie satin are different breeds of the exact same type, a coronet may appear on a pedigree. I would steer away from a silkie with a peruvian or texel in it's pedigree.

As far as price.... well it varies. A non show quality animal 10 years ago was $5, a baby animal that met standard was $15-$20, a really nice intermediate short haired pig that could be registered was maybe $30. Add a couple of competition legs and the price adjusted from there. It varied a bit by breed and selection difficulty. I once sold a pig for $125.00 but it was a very unlikely difficult color combo and very nice type.