From the time I started playing the game, I have bought numerous cards which I believe the price would increase further as more and more players join the game.
However, I noticed that staple cards no longer increase in price but have plateaued probably due to new cards entering the market that counters the old meta or act as a replacement. If you look at the price of mylor, kitty, mimosa, lir, kron, llama, etc. all their prices has dropped. A possible reason is the speculation that new summoners will be introduced soon in the Chaos Legion set.
Noone knows yet but I have stopped buying for now and keeping my cash ready. At the end of the day, cash remains the king!
What's your plan so far?