Graduate taxes would not be more affordable for the government, and would almost definitely not save any states any appreciable amount of money. Adding more taxes will hurt the average person in the US, just as many taxes have hurt people before, and subsidizing other people's ill thought out decisions is not society's responsibility.
No a graduate tax is not more affordable for governments, because a graduate tax often burdens young workers. In actuality, the graduate tax would work much like student loans currently do, but knowing the student only has to pay the money back when they have a sufficient income gives the student a disincentive to work. It is better if the student has to pay the money back no matter what, because then they will work harder.