Board Thread:Dragon City Wiki/@comment-24200782-20140504203446/@comment-25030726-20140815191214

I contacted SocialPoint about this after doing some testing. On PC, the actual amount required to add an upgrade step to the Breeding Sanctuary is 39.99%. So, if it says it will cost 10M gold, it will actually cost 14M gold. SocialPoint says they are working on it (and I believe them), but I expect it to be a while before this is fixed (partly because the motivation to fix it is probably rather low).

So, if you are going to upgrade your Sanctuary, be sure to pay for one upgrade step, then reload the page. This will correct the gold cost for the next step you want to add. If you fail to do this, the game believes you still have that extra gold you just paid for the first step and so it prices the next step according to that amount. This means the game will overcharge you for the next step and the hidden 40% cost will be calculated against this higher amount, as well. If you add many upgrade steps at once, you could be dramatically over-paying.

A side note: When I presented my findings to SocialPoint, they credited my game the amount of gold I was over-charged (the hidden 40% cost). So, if everyone who is paying gold to upgrade their Sanctuary files a report every time they add a step, then SocialPoint's motivation to fix this will increase dramatically.