Learn Croatian: Najamnina o kojoj stalno raspravljamo previše je skupa. - The rent we constantly argue about is too expensive.

This is the relative clause meaning "about which". The verb raspravljati (to discuss/argue) takes the preposition o plus the locative case. Because najamnina (rent) is a feminine singular noun, the relative pronoun koji must take the feminine singular locative form, which is kojoj.

Why is the enclitic je all the way over there after previše? Doesn't it need to be in the second position?

It is in the second position of its own clause! The sentence is split: the main clause (Najamnina... previše je skupa) is interrupted by the relative clause (o kojoj stalno raspravljamo). When the main clause resumes, previše counts as the first word of that new block, so the enclitic je naturally takes the second spot right after it.

Is there a difference between raspravljamo and svađamo se for "we argue"?

Yes! Raspravljati means to debate, discuss at length, or argue a point (like negotiating or debating the rent). Svađati se means to actively quarrel or fight with someone. Here, they are having an intense discussion or debate about the rent, not necessarily yelling at each other.

Why does the sentence use skupa instead of skupo?