Definitely a bug. Thanks for letting me know. I've updated the game to reflect the right costs however since the costs are pulled from an array at the beginning of a new file, you would have to reset the whole game.
Not to worry though, I've made a change that wouldn't require that. Just press the "End" key on your keyboard to reset the value of the Iron cost for the tick interval. Make sure to refresh the page first.
Let me know if you encounter other issues.
The fix worked (For Mac users like me, it's FN+Left arrow key) :) Thanks for the quick update and will keep you posted if I find anything else. Just trying to max out everything for now, hope you will make an update here on itch later on with more content before the official launch as I don't have a Windows machine :D