Learn Italian: Il soppalco è caro e, per di più, manca un camino. - The loft is expensive and, what's more, a fireplace is missing.

While inoltre (furthermore) adds neutral information, per di più (what is more / on top of that) adds a sense of escalation. It shows the speaker is emphasizing a growing list of negatives: first the high price, and on top of that, no fireplace!

Why is per di più surrounded by commas here?

Just like in English, per di più acts as a parenthetical phrase that interrupts the flow of the sentence to create a rhetorical pause. The commas help emphasize that this second point is a heavy addition to the argument.

Why does the sentence say manca un camino instead of il soppalco manca un camino?

In Italian, the verb mancare does not take a direct object like the English verb 'to lack'. Instead, it acts more like 'to be missing'. The thing that is missing, un camino, is actually the subject of the verb. Literally, the clause translates to: a fireplace is missing.

Could I use costoso instead of caro to say the loft is expensive?

Yes, they are interchangeable here. Caro is slightly more colloquial and frequently used in everyday conversation for things that feel pricey, whereas costoso directly refers to the literal high cost of an item.