Learn Finnish: Vuokrasimme vanhan flipperin kotiin yhdellä kympillä.

The phrase means 'for one tenner' or 'for ten euros'. In Finnish, the adessive case (-lla/-llä) is used to express the exact price in a transaction. When you buy, sell, or rent something for a specific amount, that price goes into the adessive, acting like the English word 'for'. Notice that both the number (yhdellä) and the noun (kympillä) must agree in this case.

What does the word kympillä come from, and is it common to use it for money?

The base word is kymppi, which is a very common everyday word for a tenner, a ten-euro note, or the number ten. In the context of prices, kympillä naturally translates to 'for ten euros'.

Why is kotiin in the illative (into) case here rather than a phrase meaning 'for the home'?

Finnish heavily relies on directional cases. When you acquire something and bring it to a place, the destination takes a 'movement towards' case. Because the pinball machine is ending up inside the house, Finnish logic dictates that it is rented 'into the home' (kotiin).

Why do the words vanhan and flipperin end in -n?