I have a PO Box; we've had a big issue with thefts from mail boxes in the past few years in my area- they even "fish" for mail left outside the Post office in the blue boxes. My residential mail box contains spider webs and a hand-written note from the carrier declaring it "vacant".
So... it started out at $120/year for the smallest box. I just got the latest bill; it's now $200/year, billed semi-annually. I can't remember if I got it 3 or 4 years ago but let's be kind to the USPS and assume it was 4. That's 14% per year for what's essentially "self-service".
And of course most of my incoming mail is bulk-rate junk mail that I discard before I even leave if it's not something that needs to be shredded, like a credit card solicitation.