Learn Swedish: Rutan blev krossad och reparationen var mycket dyrare. - The pane was smashed and the repair was much more expensive.

Exactly! Dyrare is already the comparative form, meaning "more expensive" (from the adjective dyr). To say "much more expensive", Swedish just adds mycket (much) directly in front of the comparative. A very common trap for English speakers is trying to translate the word "more" and saying mycket mer dyrare—but that's like saying "much more costlier"!

We learned the s-passive before. Why does this sentence use blev krossad instead of krossades?

Both forms mean "was smashed", but using blev (became) plus the past participle (krossad) focuses on the result or the change of state—the fact that the pane ended up broken. The s-passive (krossades) usually places more focus on the action itself happening. Since the rest of the sentence talks about the repair, emphasizing the broken state is a very natural choice here.

Why does the participle end in -ad (krossad) here?

When you use a past participle with bli or vara, it acts like an adjective and has to agree with the noun it describes. Because rutan is a singular en-word, we use the basic form krossad. If the sentence used an ett-word instead, like fönstret (the window), it would have to be fönstret blev krossat.