As soon as you buy them, they will crash in price. If you decide to wait, they will only go up. At least that's how it's been going for me.
No one can really predict the future, especially since these huge price drops are seemingly due to botting. The price depends on how effective they are at combating bots.
Personally as soon as I have the gold to afford 20 I plan to buy them regardless of the cost. I'm not going to try to predict the market. I'm in the same spot as you, I've got my main 6 roster in a spot I'm happy with and my next goal is working towards 5x3 for my main. If you have nothing else to spend gold on, why not just buy them now. That's how I see it, not that big of a deal if the price goes down later. I want 5x3 now, not later.