When a hot painter like the 42-year-old Belgian Luc Tuymans produces only about fifteen paintings a year, the competition to acquire them can mean prices that quickly escalate. At a just-closed show at SoHo's David Zwirner Gallery, Tuymans's nine images related to Congo -- formerly Zaire, and before that the Belgian Congo -- were priced from $70,000 to $180,000, and almost all went to museums, including unnamed U.S. institutions and S.M.A.K. in Ghent, Belgium. Just two years ago, Zwirner was selling Tuymans's paintings for $40,000 to $80,000. "It's simply such high demand and the strong secondary market," he explains, "that made Luc's primary gallery, Zeno X in Antwerp, set these prices." Just how strong is that secondary market? Only five Tuymans paintings out of approximately 400 have ever been sold at auction -- his collectors don't want to antagonize dealers and be unable to buy more.