FICTION, which used to undersell nonfiction by a considerable margin, has now leapt to the fore, and publishers are hungry for it, thirsty for it. "They isn't any fiction around," they moan. (They used to moan, "There isn't any good fiction around," but these are no longer choosey times.) It's definitely a seller's market, and a publisher has got to have what it takes to even buy a book, let alone publish it. Case in point: Agent Robert Lescher was peddling an occult potboiler, Lupe, around town last February. It had definite commercial possibilities, and a lot of people wanted it. Lescher insisted that money alone was not the consideration; publisher committment was what he and his author, Gene Thompson, insisted on. Everybody trotted out his best offer, and Random House won. This is what it took. A joint offer of $350,000 for hardcover and paperback (Ballantine) rights; a $35,000 ad budget; a tour for Gene Thompson; a first printing of 50,000; a guarantee to publish no later than September (it's actually coming out in July); a cover design and a design for the ad campaign. Have we forgotten anything? No, not even our sense of proportion, because, with the movie sale to Paramount, Lupe, is bound to pay back every cent laid out.