Use the phrase "it adds up" to talk about something that eventually costs a lot of money because you have to pay again and again.
Here are some examples of expenses that you can say "add up":
- cable subscriptions
- fees that your bank charges
English speakers usually say "it adds up" to complain about the high price of something. But you can also use this expression to talk about making money.
I tutor kids in math on the side. It's not a lot of money, but it adds up over time.