Business Secretary Vince Cable set out proposals last month for student tuition fees to be replaced with a levy based on earnings once graduates start working.
He said this would end the situation where teachers, care workers and research scientists are expected to pay the same for their studies as top lawyers, surgeons and City analysts.
But the University and College Union (UCU) calculated a university-educated nurse on average wages would pay a total of 36,871 if the Government introduced a five per cent tax on graduates' total earnings over 25 years.
By contrast, under the current system, the same nurse graduating from an English university this year would be charged 10,300 to pay off the 9,440 tuition fee loan for their three-year degree.
Even a three per cent rate of graduate tax over 25 years would work out significantly more expensive, at 22,123 for a nurse earning the average full-time salary of 29,497.
Under a five per cent graduate tax on all earnings over 25 years, a secondary school teacher on average wages would pay 46,046, a social worker 37,550, a research scientist 46,418 and a doctor 70,526.
If the rate was set at three per cent over 25 years, the same teacher would be charged 27,628, the social worker 22,530, the scientist 27,851 and the doctor 63,338.
Paying off a 9,440 tuition fee loan under the present system of funding costs the teacher 10,025, the social worker 10,272, the scientist 10,017 and the doctor 9,696.
The UCU said its research, published today, prompted fears that a graduate tax could cause a shortage of teachers and social workers.
Union general secretary Sally Hunt said: "Whatever scheme is proposed to replace fees, the Government must ensure that studying for key professions remains attractive and that the prospect of prohibitive costs over a lifetime will not put off the next generation of innovators and public servants."
Mr Cable has admitted some people could pay more under a graduate tax system.