[tosh]

Doing a Level Limit Breakthrough for monsters in your box at level 110 (in order to level them up to 111-120) uses a ton of coins. Generally, it will cost 1 billion coins to do that when the target monster is currently level 110.

If you want to see exactly how many coins you have to ensure you won't be completely depleted from doing a level limit breakthrough, you can press and hold on where it says 999,999,999+. That'll show the exact number you have. So like the game by default shows 999,999,999+ but when I hold on it, the game displays that I have 2,089,090,325 coins.

So in short, yeah there is a reason to keep hoarding coins even after you have maxed out what the game will display.