Don't know the episode, but Homer has just given Marge a gift certificate for a free spa day and she asks "How did you get it?"
Cut to a shot of Homer sitting in a car in front of a Rolls Royce dealership with the salesman. (It may say "Free spa day with test drive" on a banner, but I am getting old and my memory is not so good.)
Homer's line after this shot is: "How does this motorcar compare to, say, a train, which I can also afford."
The salesman starts talking as Homer quickly drives off. When they come back the sales guy says "... as the seat gently massages your buttocks. So, shall we talk about price?"
Homer says "No we shan't" and grabs the gift certificate from the salesman's breast pocket and dashes off, leaving the monocle spinning comically in the air.