[jachguate]

Yo creo que lo mas normal es hacer algo como:

Var
  Llave : String;

Begin
  Llave := dbGrid1.DataSource.DataSet.FieldByName('Llave').AsString;
  dbGrid1.DataSource.DisableControls;
  try
    with dbGrid1.DataSource.DataSet do
    Begin
      First;
      Suma := 0;
      While (not eof) and not (FieldByName('Llave').AsString = Llave) do
      Begin
         suma := Suma + FieldByName('CampoNumerico').AsFloat;
         Next;
      End; 
      ShowMessage('Suma ' + FloatToStr(Suma));
    End;
  finally
    dbGrid1.DataSource.EnableControls;
  End;
End;

Hasta luego.