Display data from a DataGridView column in float

I'm developing an application in Windows C # which takes the order inside a datagridview and when you press the "calculate total" button you have to add the "Cost" column. The problem is that I do not respect the decimals since it shows the whole number as an integer as the following image shows:

Instead of showing me 65.5, it shows me 655.

This is the code I use when I press the button:

private void button6_Click(object sender, EventArgs e)
        {
            for (int numColumna = 0; numColumna < dataGridView1.Columns.Count; numColumna++)
            {
                float sumTotal =0F;
                foreach (DataGridViewRow row in dataGridView1.Rows)
                { 
                    sumTotal += Convert.ToSingle(row.Cells[1].Value);  
                }
                textBox1.Text = sumTotal.ToString();
            }
        }

Try to put it in the following way:

 private void button6_Click(object sender, EventArgs e)
        {
            for (int numColumna = 0; numColumna < dataGridView1.Columns.Count; numColumna++)
            {
               var sumTotal ="";

                foreach (DataGridViewRow row in dataGridView1.Rows)
                {

                    sumTotal +=Convert.ToString(row.Cells[1].Value);
                    Convert.ToSingle(sumTotal);
                    MessageBox.Show(sumTotal);
                    }
                MessageBox.Show(sumTotal);
                textBox1.Text = sumTotal.ToString();
            }
        }

And what happens is that at the moment of doing the sum now it appears to me in the following way: