I was going to title this something much more specific, since I am not an expert on thrift stores all over the United States, but only in my corner, the northwest corner. And I am only familiar with the thrift store from the level of the shopper, not having taken an involved, inside look into the market mechanisms of such stores. But what I have seen is probably what many readers have seen as well.
When I was a child, back in the 1980's, thrift stores were very thrifty. Perhaps there is a nostalgia filter on this, perhaps I am not taking inflation into account, and perhaps I am just murky on the details, but back when I was a child, thrift stores were vast caverns full of incredible bargains. There never was a situation where price came into account when shopping at a thrift store. Shirts were fifty cents, pants were a dollar, and the books that a young and hyperlexic me would buy by the dozen were a dime or a quarter. There also seemed to be more independent thrift stores when I was a child. The big chains, which in Oregon and Washington were Value Village, Goodwill and The Salvation Army, were present, but so were a host of smaller stores, some of them being more permanent yard sales than a retail establishment.
Sometime in the 1990's, a change happened. A change that is still going on, but which has settled to the right edge of its sigma curve. Thrift stores became more expensive. They also became a bit more ritzy and better managed, but the expensive is of course the part I notice. Beyond the rate of inflation, items went up in price. I don't know if this is due to supply and demand: if the hordes of young people who believed that thrift store shopping was hip and not the last refuge of the desperately poor naturally pushed up prices. It could also be a Veblen strategy: if prices are increased, people will assume that the goods are more desirable. In any case, prices at the chain thrift stores are usually on the order of five dollars for a shirt and ten dollars for a pair of pants. Books have also gone up in price to three or five dollars. Home electronics, kitchen wares, and all the other things one might find at a thrift store have been similarly inflated.
On the other hand, thrift stores often have much better merchandise. The practice of sorting good items into display cases has made the good stuff easier to find. Also, many of the chains carry new material, usually factory seconds. The Goodwill in Portland has even opened a boutique store, where they sell their more expensive, classy merchandise.
Sometimes I will stumble into an old style thrift store, often in a smaller town. A thrift store where I can still buy a complete outfit and a week's reading for five dollars and get change back. However, as the gotterdammerung of thrift store gliterization continues, such places are more and more rare. While the conversion of the thrift store business from a haphazard project to a well-managed enterprise does make me nostalgic, it is also not totally a bad thing.