Using some basic economic principles, the idea posted in the OP will simply serve to reduce the number of people applying for STEM subjects, as many people and their parents will lean more towards the cheaper degrees as all they will think of is having less debt, thus reducing the number of qualified scientists and engineers in the future. Those who would have been pioneering scientific innovation and driving economic growth would be replaced by people who cannot find a job due to excess supply in areas such as journalism and other cheaper degree-related fields. Effectively, you'd damage the economy massively in the long run. In addition, it would be excessively difficult to determine what degrees and courses at which universities would be charged at what price - does a degree like economics class as STEM even though at some unis it involves very little maths? What about business, which often produces highly paid graduates?