Homer takes Lisa to the museum and sees the sign: "Suggested donation: $4.50"
Homer: Eh, what do you mean by "suggested donation"?
Clerk: Pay any amount you wish, sir.
Homer: And uh, what if I wish to pay ... zero?
Clerk: That is up to you.
Homer: Ooh, so it's up to me, is it?
Clerk: Yes.
Homer: I see. And you think that people are going to pay you $4.50 even though they don't have to? Just out of the goodness of their... [laughs] Well, anything you say! Good luck, lady, you're gonna need it!
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Actually the funniest part is right after that, when Lisa's substitute, Mr. Bergstrom, shows up and Homer tries to tell him that he doesn't have to pay. But the all the Simpsons quotes pages I looked at cut off just before that point.
This is from the episode "Lisa's Substitute" and I always thought Mr. Bergstrom was Dustin Hoffman, well apparently it was:
Who was Sam Etic? (29 Apr 91 USA TODAY)
If you saw Fox's `The Simpsons' Thursday, you may have noticed a new voice in the credits---Sam Enid [sic]. Enid's voice sure sounded familiar, almost like Dustin Hoffman. And gee, there sure were a lot of references in `The Simpsons' to Hoffman's first hit, `The Graduate', thrown into a story about Lisa falling into love with her substitute teacher. Shazam! It turns out Enid is none other than Hoffman, whose contract with `The Simpsons' stated that his real name could not be used on the credits or in publicity.
All this was
from here, btw. Somewhere in there, something, I think, had something to do with this thread.