Test scores for graduate and professional schools measure as much the inherent quality of the student body as they do the value of the education provided by the institution. Universities with different student compositions will have different test score results even though they may have identical quality and productivity. Graduation rate cannot drive the budget because then universities, like public schools in many places, will graduate almost everyone, regardless of performance, to ensure that the university loses no money. The difficult part of this enterprise is to tie the money to explicit measures of annual productivity. Many of the university's explicit outputs are many years in the making.
Basic research produces useful results generations from now.
The university's role in producing a better society is unquestioned, but also immeasurable and long term. To build a budget that directly links money and productivity we need measures to represent the long-term quality and impact of higher education. Most legislatures focus on the undergraduate student when looking at performance budgeting. Consider the medical services model. Medicine, like education, focuses on the individual, a patient.