The key word here is COMPETENT muffler shop.
To just replace the donut would require only a matter of a few minutes
with the right equipment and expertise, so the labor shouldn't be much,
plus the donut gasket hardly costs anything, so parts & labor shouldn be
But if the guy is inexperienced, snaps the studs, etc...now it's just
turned into a much bigger and more expensive job.
Take it to the most experienced guy you can find and you should be
able to have it quiet again for under $50.