Starting this fall, juniors and seniors pursuing an undergraduate major in the business school at the University of Wisconsin, Madison, will pay $500 more each semester than classmates. The University of Nebraska last year began charging engineering students a $40 premium for each hour of class credit. And Arizona State University this fall will phase in for upperclassmen in the journalism school a $250 per semester charge above the basic $2,411 tuition for in-state students. Such moves are being driven by the high salaries commanded by professors in certain fields, the expense of specialized equipment and the difficulties of getting state legislatures to approve general tuition increases