Try this,
private void button1_Click(object sender, EventArgs e)
{
List<int> costs = new List<int>();
//Iterate through each row in the grid
foreach (DataGridViewRow row in dataGridView1.Rows)
{
if (null != row && null != row.Cells[1].Value)
{
//Add cost value to list
costs.Add(Convert.ToInt32(row.Cells[1].Value.ToString()));
}
}
//get 50% of min value(lowest)
int result = costs.Min() / 2;
}