Forms Data Controls :: How To Multiply Gridview Rows With Textbox

I have write this code to create a gridview with 3 columns

DataTable dt = new DataTable();
dt = new DataTable();
dt.Columns.Add("ID", typeof(int)).AutoIncrement = true;
dt.Columns.Add("Name", typeof(string));
dt.Columns.Add("Price(Grouch)/Hectares", typeof(float));
DataColumn[] keys = new DataColumn[2];
keys[0] = dt.Columns["ID"];
dt.PrimaryKey = keys;
dt.Rows.Add("1", "Seaside Location", 1.5);
Session[key] = dt;
return dt;

I would like to add in this code a textbox with the quantity.

When i give the quantity i want in another textbox to have the total.

for example 2*1.5=3

How can i do that?

My huge problem is that i dont know how to take the values of the 3rd column.The value 1.5 in this example.