"Salty" in place of expensive?

Solution 1:

The opposite of "salty" in this context would be "sweet" (for the buyer), that is cheap.

"Salty" (especially in excess), implies "unpleasant," which (for a buyer) in turn implies "expensive."

Solution 2:

Although an old question I feel I might have something to add to the above answers and possibly help other confused visitors.

I don't know if 'salty' is used in the same sense in other languages, but at least here in Finland, we might use it like your friend did: "This apartment is way too small for the rent to be that salty", meaning that the rent of the apartment is too pricy and doesn't correspond to its surface area.

So if your friend was foreign (perhaps Finnish) he would have used a literal translation from Finnish to English in his sentence. It sounds a bit off in English because it's not a known or, at least not commonly, used phrase.