Hi I have the same problem here.
I have apply an workaround which is i change the values in the WebDataGrid.EditingCore.RowAdding

Let say my field with type Decimal is "Quantity", I add this code block to the RowAdding

object x = e.Values["Quantity"];
        if (x is Int32)
        {
            e.Values["Quantity"] = Convert.ToDecimal(x);
        }