The initial price is 20 gold coins. The price will increase after purchase. The gold coins required for the nth purchase are 10n*(n-1)+20.
The 4th floor store adds 2 attack points or 4 defense points each time
The 12th floor store adds 4 attack points or 8 defense points each time
The 32nd floor store adds 8 attack points or 16 defense points each time
The store on the 46th floor adds 10 attack points or 20 defense points each time
The price is the same in every store, so the later you go, the better the deal, so try to buy less in the front. It is recommended not to buy in the first area, but to buy 3 times in the second area (3 attack or 2 attack and 1 defense).