Use the price on your current item card as your savings target. After buying an upgrade, read the new price and bonus preview before planning another purchase. Do not multiply a Level 2 quote by the remaining levels to estimate the cost of reaching Level 25.
For a waiting-time estimate, enter the selected upgrade's full price, your current cash, and your online earning rate into the savings calculator. For example, the Steel Scissors upgrade above uses a target of 10,000,000 cash; the calculator subtracts your balance to find how much more you need.