Learn Finnish: Toisin sanoen matka on liian kallis meille. - In other words, the trip is too expensive for us.

It literally translates to 'saying otherwise'. Toisin means 'otherwise' or 'differently'. Sanoen is the second infinitive form of sanoa in the instructive case, which translates to 'by saying'. Together, they form the fixed expression for 'in other words'.

Why use the allative case meille (to us) here instead of a postposition like meitä varten (for us)?

While meitä varten does mean 'for us' in the sense of something being made or reserved for someone, it doesn't work here. In Finnish, when you state that an adjective is 'too' something for someone's limit or perspective (like 'too expensive for us' or 'too heavy for him'), you use the allative case (-lle).

Could I use liikaa instead of liian to mean 'too' in this sentence?

No, because you are modifying an adjective (kallis). You must use liian before adjectives and adverbs. Liikaa means 'too much' and is used when describing amounts with nouns (like liikaa rahaa, 'too much money') or when modifying verbs (like maksaa liikaa, 'costs too much').