I think there is some problem in your code. Write something like this
decimal mon = 0;
decimal comPrice;
for (int i = 0; i < dgvProducts.Rows.Count; i++)
{
try
{
comPrice = (decimal)dgvProducts.Rows[i].Cells["cPrice"].Value;
mon += comPrice;
}
catch(Exception e)
{}
}
lblPrice.Text = mon.ToString();