They are normally brazed or soldered on.
Look closely and see if there is any signs of it being soldered. Also there should be a little peg sticking out of the barrel that fits into a matching hole in the bottom of the sight.
If there is no or little signs of the soldering I'd call Ruger and see if they'd send you a shipping label and cover the whole charge.
After all, it's their boo boo.
If they won't do it and you've got no gunsmith handy you can try LocTite Black Max adhesive. It's really black and it will glue steel and lots of other things together like nobodies business. I've fixed a number of things with it that nothing else would fix.
It's not cheep but you can get it from Midway and other places.
Joe