A USPS PO Box, for the smallest size, is around $50 per year though it varies widely, because each local USPS PO sets their box rates.
http://www.usps.com/receive/business...boxservice.htm
And then click on the "Find the fees near you." link part way down the page, enter your zipcode and it will show you all the box rates for the local POs in your area.
As another poster pointed out, the USPS will, in many instances, reveal street address information to those who request it.
Getting a box at a private mail place like the UPS Store may be better in regards to privacy, but not necessarily, because agreements vary widely from location to location (ie. most all UPS Stores are separately owned and operated), and more to the point, such services are considered "Commercial Mail Receiving Agencys" (CMRA) ... and thus even when applying for a private mailbox, you'll likely be asked to fill out a US Post Office PS 1583.
Ron