Leaders of the two national governors organisations say giving teachers half a school day a week for marking and preparation will add 10 per cent to budgets.
The Chancellor has promised that education spending will rise by 6 per cent per year over the next three years but governors say they have had no indication that enough of that cash will reach schools.
Heads have already said they will pull out of the historic workload deal if the cash is not there to back the changes - planned over the next three years.
The offer of half a day a week "non-contact" time for teachers by 2005 is key to the workload deal and a crucial reason why it has won the support of two of the three largest classroom unions.
Ministers believe highly trained assistants (see story below) can reduce the cost of implementing the deal. They expect the cost of workload reform to be pound;1 billion a year by 2005 But governors say more money will be needed if pupils are not to lose out.
Jane Phillips, chair of National Association of Governors and Managers, said that teaching assistants with higher-level qualifications were scarce and more than one assistant would be needed to replace a single teacher.
"It is horses for courses but it must be good quality and you get what you pay for. The workload issue must not be a way of employing cheap labour," she added.
The National Union of Teachers has refused to sign the deal because it believes it could lead to cheaper staff replacing teachers.
In a letter in today's TES, Jane Phillips and Neil Davies, chair of the National Governors' Council, plead for a meeting with schools standards minister David Miliband to resolve funding problems.
They say he has ignored the 370,000 volunteers who help runs schools. They are upset that governors have been sidelined even though they are responsible, with heads, for setting budgets and staffing levels and so will be at the centre of delivering the workload reforms.
In their letter, the pair say: "The signal being sent out at the moment implies that 'governors are unpaid volunteers and we have no price tag, therefore we have no value'."