There Ain't No Such Thing as A Free Lunch.
Very old slang, dating back to before ain't
became a legitimate English contraction.
I means that a prospect is too good to be true(TGTBT
), in that you are being offered something for nothing.
Nate: Your eMail says Bill Gates will give you $250 for each person you send this to that sends it to someone else.
Uncle Dick: TANSTAFL.