Thanks, Jonathan. It all makes sense now. Fit Saltypetra’s comment about rounding as well. Great job coming up with the formula before this post explaining it even existed. So awesome!!
Also, I didn’t expect the most spooky thing in Halloween to be not ghost, but complicated math. 
So, tonight, my plan to spook myself is not just watching horror movie now, but include making a formula to calculate the cost of upgrading Treasure Hoard in any point, to predict gold requirements in customized multiple upgrades. Stay tune! ^^
Edit : Halloween’s math ghost is proving to be much more scary than I thought, so it take me 2 night to deal with it. But it was defeated now, and the result is available here!