I overlooked this comment @pokermaria, so late reply here

https://www.soraredata.com/ is where you can get price charts of the past.

If anything, I would say now is the time to stack up and buy some players as prices came down a lot while the game itself is fun and still quite undiscovered I would say. The tricky thing remains the card inflation each season. This kind of translates in it getting harder to win prizes (aside of of cap240 leagues which I like the most to compete in). Knowing your league is really a requirement in order to strike some good deals and it's imo always better to try and get players really cheap when they don't play or are injured instead of buying to expensive ones who run the risk of injury or something else that makes the price tank.

The changing of teams goes pretty fast I believe, it takes until the start of the new season for new cards to get printed and the old ones lose -5% bonus. (this doesn't matter in the cap240 league as bonuses don't count)

No clue on the popularity in asia.

If you are able to pick young players that get end up getting their chance and perform well you should be able to do good. I managed to pick up Vermeeren at 7.30€ who now goes for 40€ and has a bright future ahead of him now at age of 18.

I would say best way to play now is to either stick with trying to pick young or very undervalued players (even though that is quite hard since many that don't play but are known to be big talents are already quite expensive) and/or play the cap240 league as there it's very clear how to calculate the potential ROI. 5$ each time you get 250 points in Limited and 50$ each time you get 250 points in Rare with potential card prizes if you reach very high)

Good Luck!