Your code  is totaly unoptimized. You get data for each item, and header and footer. You should put everything inside if statement and don't forget to cast types. when calculating price and displaying it in Text property of TableCell. It's strange that you don't get any casting errors.

Your variable indexInDataGrid is of the same value when calculating sum. You don't actually calculate  on each row  of the DataTable. "foreach" statement should be excanged with "for" statement with counter to calculate the sum.

for (int i = 0; i<myTable.ROws.Count; i++)
 sum  += (Double)myTable.Row[i]["totaleprijs"];