No, this isn't about someone weeping over a vacant lot. The title refers to the ending of the book, when the Pierce Inverarity's collection of stamp "forgeries" is auctioned off as "lot 49" (6.128).
In the final pages of the novel, Genghis Cohen explains to Oedipa that "crying" is the technical term for how an auctioneer calls out a sale at a formal auction. As the day approaches, the mystery around "lot 49" begins to grow, and Oedipa hears that an unknown bidder has surfaced and will attend the auction.
"The Crying of Lot 49," then, is the moment to which the entire novel builds. It may be the moment of revelation (in which Oedipa finds out that the Tristero conspiracy is real), or it might be the moment at which Oedipa realizes that she is the victim of an elaborate practical joke. Or it may be yet another twist in the huge mystery that is this deeply strange book. Or hey: it might be a knock-knock joke.
The point is, we never learn what happens at the crying of lot 49. The novel ends—in a really cruel twist—with Oedipa waiting for the auctioneer to start the bidding.