I think the main thing is that Sam is extremely experienced in using the flight harness, but he was also part of an entire USAF PJ unit using them at least a decade before this film. Assuming he’s not the only survivor, and they weren’t the only unit, there must have been at least dozens of such operators and dozens of such harnesses, which have only got more advanced in the last decade.
How expensive are they? Who knows, but Torres got one*, no questions asked, just because he’s a promising young USAF officer who’s Cap’s best pal. He’s not a superhero, we have no reason to think he’s unique or even exceptional, he’s just a great pilot and hacker.
*Torres says “I make your old suit look good” but since Sam’s last suit got wrecked in FatWS and it didn’t look much like the one Torres has, I’m going to assume it’s a similar model and not the same one.