[xdessx]

Код:

if ((ComboBox1.text = "Масква") or (ComboBox2.text = "Масква")) then begin
 gorod1 := 100;
 a := 1;
end;
if ((ComboBox1.text = "Саратов") or (ComboBox2.text = "Саратов")) then begin
 gorod2 := 300;
 a := 2;
end;
if ((ComboBox1.text = "Питер") or (ComboBox2.text = "Питер")) then begin
 gorod2 := 400;
 a := 3;
end;

if (a = 1) then
Label1.Caption := IntToStr(gorod2 - gorod1);
if (a = 2) then
Label1.Caption := IntToStr(gorod3 - gorod2);
if (a = 3) then
Label1.Caption := IntToStr(gorod4 - gorod3);

Ну дальше сами...