Learn Finnish: Auton korjaus maksaa paljon. - The repair of the car costs a lot.

It is derived from the verb korjata (to fix / to repair). By taking the verb stem and adding the suffix -us, it creates a noun representing the action of doing that verb. So, korjaus means "the act of repairing" or "the repair".

Why is the suffix -us used here instead of -os?

The -us/-ys suffixes generally describe the action or process itself, which fits the act of fixing a car. The -os/-ös suffixes are typically used for the tangible result of an action, such as leivos (a pastry) from the verb leipoa (to bake).

Why is auto in the genitive form (auton)?

When you use a noun that describes an action (like korjaus), the target of that action takes the genitive form. While English says "the repair of the car", Finnish literally phrases it as "the car's repair" (auton korjaus).

Doesn't the verb maksaa mean "to pay"?

Yes, but maksaa means both "to pay" and "to cost". You can easily tell which one it is from the context. Because the subject here is an inanimate process (auton korjaus) rather than a person making a transaction, it translates to "costs".