Whats the deal with path of exiles player base? According to steamcharts it fluctuates like crazy. Does the game have a high turn over rate?
Game has seasonal leagues, each league lasts for 3 months.
New league is pretty much where lots of people come back to the game to try out new league mechanic, new skills/gems and maybe new unique items. Not all leagues are great, some leagues can be quite considered bad "according to reddit". However, usually the season that starts around near the end of each year is the biggest one, due to big updates endgame and can have league mechanic be made permanently into core game.
About big amount players dropping off, it usually starts after a month. Because, even if you are really really casual, you can reach endgame within two weeks, then basically doing endgame to min-max your build. A month is probably enough for playing new season and new league mechanic is also happens during leveling, not just end game. Depends on how well liked the new mechanic, more players will stay or stop playing.
The ones that continue playing the game are usually the ones making multiple builds/characters playthrough.
It's F2P so I'd imagine it gets a bump every time new stuff is added and steam puts it on the frontpage.
Then people play it and realise it nickel and dimes like every other F2P so they dump it.
PoE microtransaction shop are pretty much convenient stuffs and typical fashion cosmetics thing.
Convenient stuffs are required (must have) to have are more stash tabs, premium stash tab (for easier to trade) and currency tab (the game relies on these currencies to do trading between players).
If you are a casual, playing on one character only, one premium stash tab and currency tab are all you need. Unless you are playing this game a lot (multiple characters), whole seasons or being streamers, then you probably need to buy more stash tabs and a few more specific convenient things (maps, essences, fragments, divination cards).
Shop price, not that bad overall. A casual player need only to spend like 10-20 dollars at most.
There are also sales off now and then for microtransaction shop as well.