The market decides all of this. I bought a handful of flies in TCO last time in. Needed a few Sulphurs, and they had a kinda unique Slumpbuster pattern and Hopper kinda pattern that I liked and wanted to try a couple of. Yeah, even though the prices are posted by the flies I was a little surprised by how much a dozen flies ran. I had already picked them up, and had the kid ring them up, so I paid for them anyway. Probably won’t moving forward though. Or just limit myself to one of a new cool pattern I want to try. This was TCO is SC, and I like the store, and have had nothing but good experiences in there. But yeah, $3-$4 for a fairly standard issue dry or nymph pattern is steep. And $5-$6 for a fairly simple streamer like a Slumpbuster is kinda getting silly.
Tie your own, or buy your bulk at one of the online vendors for $1/fly or whatever. You can get 3-4 for the price of one in a shop, and yes they don’t last as long, but collectively 3 or 4 them absolutely do hold up longer than one shop tied fly. I don’t notice any difference in their ability to catch fish, just more so how long they last before they start to unravel. As to dries, FWIW, Catskill ties from the online outfits seem to hold up better than the parachute style flies. On the parachutes the hackle seems to unravel pretty easily. Not sure if there is a solution to reinforce those somehow before you start fishing with them.