This is untested, but should work:
lblTotalPrice.Text = Convert.ToString(
dataGridView1.Rows.Cast<DataGridViewRow>()
.Select(x => Int32.Parse(x.Cells[6].Value.ToString(),
NumberStyles.AllowCurrencySymbol))
.Sum());
It takes the 6th column of your DataGrid
, which should be Cost
, then converts the string value back to a number and adds them up.
Execute this code as needed to update the total, such as when you click the "Add New Room" button.