The British Medical Journal recently estimated that PFI operators can expect rates of return of 15-20% a year.
The bill for large NHS PFI schemes already completed, under construction, or approved, currently comes to almost £14bn.
GMB general secretary John Edmonds dismissed government claims that private-sector involvement would bring extra cash into the health service.
He said: "The government's privatisation plans are effectively creating a new breed of NHS loan shark at the taxpayer's expense.
"Who on earth can justify paying rates of return to the NHS privateers of 25% in the name of greater efficiency?"
Edmonds added that private contractors were taking more than £1.5bn from the NHS.
A poll commissioned by the union found that one person in nine supported the government's proposals to extend private-sector involvement in public services.
Hospital PFI deals will face increased scrutiny from parliament's influential select committee on health. The committee has asked health secretary Alan Milburn to explain how far privatisation in the public sector will go.