In all the cases I can think of, the pay of an individual teacher was based on his reputation and the prestige / size of his classes and the school, rather than which of the arts he was teaching. This is made particularly clear in Tangheroni's contribution to the official history of the University of Pisa (Storia dell'Università di Pisa, 2000), in which the numerous teachers and their respective salaries differed considerably, even in the same year. In 1458, Landino da Pratovecchio was paid 100 florins a year to lecture on oratory, poetry, and philosophy, while Giovanni Argiropulo was paid four times that for lecturing in the same subjects (pp. 100). Tangheroni's specific examination on these masters demonstrates that, as with everything else, the pay awarded to university teachers depended upon the priorities of that university: if they wished to employ prestigious teachers, huge amounts of money would be spent to attract the most prestigious scholars available; if they wished to strengthen their arts course or their theological faculty, the funds would be directed accordingly. It should be pointed out, however, that as a considerable number of arts masters at larger universities were also students of the higher faculties, they would not be getting paid anywhere near as much as these famous names, or even as much as their less famous teachers in the higher faculties in which the studied. Feel free to ask follow-up questions.