Nothing like a 7-by-9-foot cell to get that creative Juice flowing especially when the payoff may be in the millions. Although it's been reported that O.J. Simpson will receive $1 million for proclaiming his innocence in his prison- dictated tome, I Want to Tell You, an inside source says the sum is really closer to $4 million, a figure being denied by publisher Little, Brown. Roger Straus, president of Farrar, Straus & Giroux, thinks the whole thing is ''revolting! The lawyers were obviously pushing for him to do it.'' Simpson reportedly almost broke agreed to the deal before Jan. 1, skirting California's newly toughened Son of Sam rule, which would place royalties from the book in a trust fund for the victims' families if O.J. is convicted. But Will Schwalbe, editorial director of William Morrow, asks, ''Until proven guilty, how can we stop innocent people from writing books? The only question is, Who should profit?'' Not whether they will profit. Says Straus, ''Oh, (Little Brown will) make back their money. But do they have to make money that way?''