I’ve not been as lucky as Brad on this one. I think a lot of this depends on the area you are in, and the luck of the draw on the postmaster for that area. I’ve always had to pay for them, and you are right – they are INCREDIBLY expensive, not to mention the concrete slab you have to pour.
We did have one park that was allowed to get rid of the mailboxes altogether (we did not put that deal together, a former owner did). He required every tenant to get a P.O. Box in town. It seemed to work fine, and even brand new mailboxes are still uglier than none at all, so it was in the park’s best interests. You might try that sales pitch on your postmaster. That would be free to you.