This calculator is arithmetic on a table, not a live mixer. Enter quantities from your own Stop The Tsunami! session. It will not click the shop.
Upgrade payback
Machine payback
Estimate how many surviving waves it takes for a machine purchase to earn back its coin cost. Enter the shop price and the extra coins you think that machine adds on each later wave.
Payback cycles = upgrade cost ÷ extra profit per cycle, rounded up.
Worked example only. The game does not publish a coin-per-wave table, so enter the coin amount you actually received. Upgrade Cost is the price shown in the shop. Extra Profit Per Cycle is the extra coins you expect on each later wave because the machine lets you finish a bigger defense. Cycle Minutes is how long one prep-and-wave loop takes you.