Shouldn't be any more than $200. My C-280 cost $185 IIRC.
The 320 is pretty much the same engine, just a little more displacement. If the dealership wants more, run to your nearest independent Mercedes mechanic. Have a look yourself. If the belt shows wear in the form of significant cracking or missing chunks, it's definitely time for a new one. A screeching belt doesn't necessarily mean it needs to be replaced, just that it needs to be retensioned. If it looks old replace it, if not just retension it.
And, yes, the belt is not a timing belt. It's for ancillaries.
The serpentine belt can be replaced very easily for about $30. I used a Dayco belt. I have written a procedure before in this forum. It is for an S420, but, perhaps it is similar to the S320. The Dayco belt is better than OEM I think. Regardless, the procedure is very simple. Literally a 10 min job. BTW I was quoted $45 for labour to replace the belt. So total cost should not exceed $75.
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