Forms Data Controls :: Modify Gridview Cell Value By Dropdownlist Inside

I have this question for a gridview that ai use for a shopping cart. On my first 4 collums is data from my database cart (the 4'th collumn contain the price), on the 5'th collumn is a dropdown list (having 1 to 30 values). what i want is when i change the value in dropdown list the 6'th collumn to be updated to the value of(dropdownlist value * price). on page_load i have this

GridView1.DataSource = clasa.my_chart_bike(Convert.ToInt16(Session["id"]));
GridView1.DataBind();
on GridView1_RowDataBound i have this
if (e.Row.RowType == DataControlRowType.DataRow)
{
string t=((DropDownList)e.Row.Cells[4].FindControl("DropDownList1")).SelectedValue;
e.Row.Cells[5].Text =Convert.ToString( Convert.ToInt16(t) * Convert.ToInt16(e.Row.Cells[12].Text));
}
GridView1.DataSource = clasa.my_chart_bike(Convert.ToInt16(Session["id"]));