Learn Finnish: Lippu saareen maksaa kolmekymmentäseitsemän euroa.

Saari is in the illative case here, which means "to" or "into" a place. To add the illative ending, the basic word saari changes its final -i to an -e- to form the stem. Then we add the ending -en, making it saareen ("to the island").

I thought the verb maksaa meant "to pay". Why is it used here?

The verb maksaa is versatile! Depending on the sentence, it means either "to pay" or "to cost". Since a ticket can't pay for things, it's clear from the context that here it means the ticket costs 37 euros.

Is it normal to write a long number like kolmekymmentäseitsemän as a single word?

Yes! In Finnish, numbers from 11 up to 99 are always written together as one continuous word. This word literally squishes together kolme (three), kymmentä (tens), and seitsemän (seven).

Why does the word euro have an -a at the end?

Because it comes after a number! In Finnish, any noun that directly follows a number greater than one must be put into the partitive case. Adding -a to euro turns it into euroa.