DataGridColumn SortMemberPath on MultiBinding
SortMemberPath is expecting the name of a property (e.g. "TotalDollars") not an individual computed row value. Think of it like the header, you set it once for the whole column. Your converter would be returning a number like 15 where SortMemberPath wants a binding path string.
Two options that come to mind:
Provide a computed property on your backing object (e.g. "AveragePrice") and bind to that. No converter or sort member path necessary.
public double AveragePrice{ get { return TotalDollars / NumberToDivideBy; }}
Hope it helps. :)