Hi,
Use the row databound method of gridview and into this method find the control of the gridview cell and then multiply the textbox value with the cell value.
void GridView_RowDataBound(Object sender, GridViewRowEventArgs e)
{
if(e.Row.RowType == DataControlRowType.DataRow)
{
e.Row.Cells[1].Text = (Convert.ToInt32(e.Row.Cells[1].Text) + Convert.ToInt32(textBox.Text)).ToString();
}
}
In the gridview add the below event.
<asp:gridview id="GridView" onrowdatabound="GridView_RowDataBound" runat="server">
//Your gridview code
Hope this will help you