I don't remember exact details, but I think I heard somewhere that when filming the original Dawn of the Dead in 1978, the shopping mall allowed Romero to use it for free. The things was, however, he could only shoot after the mall closed, and had to be done shooting and have everything cleaned up by the time the mall opened the next morning.
So he got the location for free, but they only worked by night.
Or so I've heard.
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