procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
var 
  vrPos, vrLength, vrSelStart: byte;
const
  I: byte = 1;
begin
  with Sender as TEdit do
  begin
    vrLength := Length(Text);
    vrPos := Pos(',', Text); 
    vrSelStart := SelStart; курсора
  end;
  case Key of
    '0'..'9':
      begin       
        if (vrPos > 0) and (vrLength - vrPos > I) and (vrSelStart >= vrPos) then
          Key := #0; 
      end;
    ',', '.':
      begin      
        if (vrPos > 0) or (vrSelStart = 0) or (vrLength = 0) then
          Key := #0 
        else
          Key := #44; 
      end;
    #8: ; 
  else
    Key := #0; 
  end;
end;