Thanks for being patient while I got back up to speed after my time out of the office and did some investigation into this issue.
We haven’t made any retroactive changes to the cost to increase skills without telling players first. Whenever we’ve done this in the past, we’ve been transparent about it, letting players know and explaining the reasons for the change.
The gold cost for skill upgrades scales as the level of the skill increases. It steps up at levels 3, 4, 43, 87, and 131. There are many places in the game where costs scale up - campaign stamina, hero stars, etc. These are not things that we announce ahead of time - they’re part of the system within the game - the natural progression.
There is a balance between what things cost in the game and the supply of resources the game provides. We know we don’t have that balance perfect for gold, and we’re working to address it. Recently, we did increase the amount of gold and stamina that’s available in the game (not just in solo deals) to help with this. We’re always working to fine tune this balance to ensure the game experience is both challenging and rewarding.