User blog comment:The Arrival/Batman: Arkham Knight Mysteries/@comment-5795364-20140707170745/@comment-50.151.180.255-20140731151246

That problem Gordon was referring to in Harley Quinn's revenge was about Jokers initiating a contingency plan, whoever found his body would get X amount of money(I don't remember the amount).

If u read the Batman Arkham City endgame comic-it reveals Batman and Gordon burned Jokers dead body, so the Joker is definitely dead.