in checkbox_checked() event, write the following code
int amount=0;
for (int i=0;i<grid.rows.count;i++)>
{
CheckBox chkBox=(CheckBox)grid.FindControl("chkBox");
if(chkBox.Checked)
{
amount+=Convert.ToInt32(dtGridData.Rows[i]["Amount"]);
}
}
lblTotal.Text=amount;