You can mail it to apple for $7 or so, but that doesn't factor in not having the phone for a few days (I assume the independent shops will do it same-day). From a pure price perspective, that may or may not be on par with what the local shops charge. It's lower than the price I quoted, and other places seem to charge around $80 or so, but smaller shops may charge less (for whatever reason, most places want you to fill out a form or call to get a quote).
To be clear, I don't think Apple's software lock is a good thing, and I wish their phones (and other phones that copy them) had more user-serviceable parts. But availability aside, I don't think their in-store prices are unreasonable, given the work required, consumable parts (seals and glue), and the retail price of decent batteries.