Protecting schools from real-terms cuts in England would mean spending the equivalent of an extra penny on the basic rate of income tax, says the Institute of Fiscal Studies.
The financial think tank says maintaining funding at current levels would mean raising spending by £3.7bn.
Head teachers have been warning of job losses from budget shortages.
But Luke Sibieta of the IFS says: "A promise to protect schools from cuts will not come cheap."
School funding has been an election battleground - with opposition parties highlighting budget shortages.
Teacher job losses
In a blog for Schools Week magazine, Mr Sibieta says preventing this funding gap would mean a commitment to "significant additional public spending" - worth a penny on income tax or 3% of the NHS budget in England.
He says it would cost £2bn to protect per pupil spending in the next four years and another £1.7bn to cover additional costs facing schools - representing a 10% increase in the school budget.
But head teachers, protesting about inadequate budgets, have published their own research into job losses caused by funding cuts.
Heads in more than 700 schools say more than 3,400 posts have already had to be closed, including more than 1,000 teachers.
The heads in 14 counties, across the south and east of England, issued a joint statement: "Head teachers have been warning of an impending crisis in their schools. The crisis is no longer expected, it is under way."
They say that after months of warnings about cash shortages, financial decisions about staffing for September have already been taken.
In these schools, 1,107 teaching posts together with those of 1,488 teaching assistants and 820 administrative and support staff have been lost.
Record spending levels
There are more than 24,000 schools across England - and this count of job losses by head teachers covers the current situation in just over 700 schools.
Among heads in these schools, 43% thought their budgets were "inadequate" and 49% defined their financial position as at "crisis point".
Only 8% thought funding levels were adequate or good.
West Sussex head teacher Jules White, who has co-ordinated a campaign on school funding shortages, said: "It is clear that devastating cuts to staffing and educational provision are occurring in thousands of schools across the country.
"In turn, children's' educational opportunities are being blighted by the loss of thousands of teaching and teaching support posts."
In the general election campaign, Sarah Olney for the Liberal Democrats said funding cuts meant "fewer teachers, larger class sizes and schools unable to afford basic repairs".
She accused the Conservatives of "splashing out" on "divisive pet projects like free schools and grammars, while the majority of schools see spending per pupil fall".
Labour leader Jeremy Corbyn told a head teachers' conference that he would tackle the funding gap
"I am determined to lead a government that will give the priority it deserves to education," he said.
But the Conservatives have defended their record on school spending - saying that school budgets have been protected and are at record levels.