The Hoax

In 1971, publishing executives at McGraw-Hill express an interest in Clifford Irving's novel, Rudnick's Problem. Fake!, his previous book about art forger Elmyr de Hory, had sold poorly. Irving believes he has a breakout work, but the publisher decides against releasing the book after a Life editor deems it unsatisfactory.
Vacationing with his friend and researcher Richard Suskind, Irving is ejected from his hotel at 1 a.m, after eccentric billionaire Howard Hughes arrives and demands the entire building be vacated. Returning to New York City to meet with his publishers, Irving finds he has been reduced to meeting with an assistant. Irving storms into the board room, says his new project will be the "book of the century", and threatens to take it elsewhere. He struggles to come up with a topic fit for his grandiose claim. Seeing a cover story on Hughes, he decides to make him the subject.
Irving tells McGraw-Hill that Hughes has recruited him to help write his autobiography, and shows forged handwritten notes from Hughes as proof. Handwriting experts confirm the notes as genuine, and the publishers strike a $500,000 deal for the book. Irving believes that the reclusive Hughes is unlikely to sue him, and that his eccentricities can be used to deflect any denials of authenticity for the book.
At the time Irving is having marital problems with his artist wife Edith; he has had an affair. Irving assures Edith he'll be faithful as he leaves to begin research with Suskind. To fool experts, the two men devote days to studying documents pertaining to Hughes. They illicitly obtain a copy of a draft biography of Noah Dietrich, a retired Hughes aide, which provides details that add to the apparent authenticity of their work. Irving recites passages into a tape recorder while in character as Hughes, dressing like the millionaire and adding a mustache during these sessions.
As work on the book progresses, Irving receives a box containing scandalous material about questionable dealings between Hughes and President Richard Nixon. He believes that Hughes sent the package and convinces himself that Hughes wants this damaging material included in the book, as a sign he supports the work.
As the publication date draws near, Irving elaborates his hoax, staging an "aborted" meeting between Hughes and the publishers. Hughes has officially denied that he was involved in the work, but the McGraw-Hill executives are convinced it is genuine. They begin to think it will be a bestseller and Irving works to gain larger payments for himself and (purportedly) Hughes. Irving and Edith concoct a scheme for her to deposit Hughes' check, payable to H. R. Hughes, into a Swiss bank account, using a forged passport with the name Helga R. Hughes.
The continuing drama makes Irving increasingly paranoid. He has alcohol-fueled fantasies about being kidnapped by Hughes' people. His affair with Van Pallandt continues, and the pressure of keeping up a pretense of fidelity with his wife adds to his stress. In what is implied to be a favor to Nixon, Hughes goes public via a televised conference call and denies any knowledge of Irving or the book. Irving is arrested and agrees to cooperate if Edith is granted immunity. At a press conference, a government spokesman announces Irving, Edith, and Suskind have received short jail sentences.
An overheard radio report details a sudden wave of legal decisions in favor of Hughes. Irving believes this indicates that his book had been used to place the president in debt to Hughes. A fleeting scene set inside the Nixon White House suggests that Nixon's preoccupation with Hughes led directly to the burglary and wiretapping of Democratic Headquarters at the Watergate Hotel. Historians and political analysts dispute this.[2]

Question: The publishers struck how much of a deal for the book?
Answer: $500,000