What type of buckle is it? On webbing or piping? If it's webbing you can get a sternum strap with a split so it is removable/easy to replace the old one. With piping it should be possible to force a new one on with a screwdriver.
I just replaced a broken buckle on a sternum strap recently and all I did was unpick the stitches on the webbing (both male and female sides), grab a buckle from my spares box and had a cobbler sew the straps again. Cost me all of £1 but that's with middle-east prices.
If it's on an Osprey pack, Osprey will send you a replacement for free. But that sounds too much like an easy solution so perhaps it's not on an Osprey pack...