Resale prices for anything vary over time with both rarity and demand, and lots of things change demand both up and down. Mid century modern items were worthless thrift store shelf warmers till it became popular and "in", and prices went up. Once the next decorating fad hits it will fade. Actual antiques ride many many up and down swings in price. The only people that make money in antiques are able to find and acquire things that aren't in demand now at low prices, or to travel and get things where they're plentiful and cheap and take them to where customers are.
Figures go up and down the same way, based on demand. OG Avengers Hawkeye was incredibly valuable in between A1 and Ultron as it was under produced and sold out before people realized the movie was great and they wanted the whole team. It was not a particularly good figure. Once Ultron Hawk came out the price cratered, as people suddenly had an outlet to buy a Hawkeye for their team, and only moreso as more movies have come out as people have more options and better sculpts. The flipside is Avengers Loki. Stayed at about retail for a while, once Dark World Loki was announced, Avengers Loki shot up in price, purely because DW Loki was noticeably a worse figure, and everyone who decided to wait for the next one panicked. There's probably not going to be another classic Loki figure, but if it gets reissued and improved watch that price drop.
Its been the typical case that older figures drop in price, its just usually because the same character is in a new movie every 2 years, and the quality and engineering level has been going through the roof- no one is going to fight over Avengers 1 Thor when there's Ragnarok or Infinity versions. Or the new diecast original 7 Iron Man armors that make the originals look faded and off model. But at some point these characters wont be in the films anymore, and one particular version will end up being the Captain America or the Doctor Strange to have, but if that figure is out already or if we haven't seen it yet is anyones guess.