Introducing the two main elements of single status could cost more than £247 million, adding nearly 2% to the combined manual and white-collar pay bill, according to rough estimates being used by the negotiators.
'The calculations show that single status will add to the pay bill,' a GMB spokeswoman said. The union wants to avoid a below-inflation pay rise for the fourth year running, and says pay should be negotiated before single status.
The other two main local government unions, T&G and Unison, want talks on pay and joint conditions to run in parallel, and hope for an agreement on single status by Christmas.
Cutting the 39-hour manual working week to the 37 hours worked by white-collar staff could cost 1.4% of the pay bill, (£182m).
These figures are an indication of the national cost of single status. The cost of implementing the reforms will vary widely between councils.
Unions do not want single status to affect next year's pay rise.
'We intend to keep the negotiations on the pay increase for next year separate from single status. We've got to address the issues on their own merits,' said Unison head of local government Keith Sonnet.
Employers say there is no extra money for single status, and it will either have to be funded from money which would otherwise go into the pay increase, or it will affect jobs and services.
T&G national secretary Jack Dromey said: 'The status quo would still cost because change is inevitable. There will be a common grading structure because on equal value the only question is do we get there by negotiation or litigation. The change is best managed by negotiations.'
Mr Dromey said the final cost of single status is impossible to predict because it will depend on what is agreed and over what period it is implemented.
Employers want changes introduced over three years to spread the cost. Unions are hoping for a shorter timetable.