"spontaneous failure" isn't a diagnosis. Spontaneous failure of what, exactly? Nerull's guess is a good one (shattered ball bearing), but you should have the shop open up the seals to check it out. It should probably be under warranty.
$50 is not totally unreasonable for the cost of good steel bearings and installation. It's a 5 minute job though, so see if you can push them on the timetable, and bring the mech a sixpack/bottle of his/her favorite as a thank you.