[PMCarey]

That's from "Duck Soup". The full exchange is:

Groucho: What do you get an hour?

Chico: For playing, we get-a ten dollars an hour.

Groucho: I see. What do you get for not playing?

Chico: Twelve dollars an hour. Now for rehearsing we make special rates. That's-a fifteen dollars an hour.

Groucho: And what do you get for not rehearsing?

Chico: You couldn't afford it. You see, if we don't rehearse, and if we don't-a play, that runs into money.

[A Jovial Cad]

LMAO! Thanks for bringing that memory into focus, I'd forgotten the particulars. Still one of the classic punch lines of all time.