How about, "If you'll forgive me for not offering a discount, I'll forgive you for asking for one"?
Personally, I would not use such a sign. Too antagonistic. If they ask for a discount, just smile sweetly and say, "That won't be possible."
Then, depending on how snarky you feel, you might suggest they go shopping at Walmart where they will probably find something in their price range. (just kidding)