When the Coalition of BC Businesses waded into the teachers' dispute last week, it once again inflamed debate over how much a Supreme Court judgment ordering a return to pre-2002 class sizes and staffing levels would cost the province.

The coalition, which represents 500,000 employees and 50,000 employers, called the "$2-billion ruling" a "threat to taxpayers." The coalition said if the ruling is allowed to stand, the government will be forced to raise taxes, cut services or return to deficit spending.

(Read up on the issues and history of the education labour dispute with our explainer Q&A.)

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But where exactly did the $2-billion figure come from?

It turns out that number, which has been widely cited, isn't an accurate estimate of how much it would cost to return to those pre-2002 levels. Estimates actually range from $200-million to $1-billion annually.

The contested figures are major sticking points in the bitter labour dispute between teachers and the province that has cut short the school year by two weeks and shut down summer classes.

The B.C. Supreme Court has twice ruled the province acted unconstitutionally by stripping the language around class size and composition from teachers' collective agreements, removing their ability to address their working conditions during bargaining.

In a January ruling, Justice Susan Griffin quotes a government representative who had said reinstating the provisions would cost $500-million in retroactive liabilities and $200-million to $275-million going forward. That's far less than the $2-billion figure often cited.

The $200-million number comes from an estimate done back in 2000 by Treasury Board, but nobody knows how it was calculated, said a spokesman for the B.C. Public School Employers' Association (BCPSEA).

The same court ruling includes an estimate that the changes would cost the Surrey School District $33-million annually. Because the district represents around 10 per cent of the province, some have speculated that the total cost of the changes would be about 10 times the Surrey figure – roughly $330-million.

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It's a number that Jim Iker, the president of the B.C. Teachers' Federation (BCTF), doesn't contest. But Mr. Iker does object to the government's more recent estimate, which is in the realm of $700-million to $1-billion.

BCPSEA said it arrived at that range by creating a spreadsheet of every classroom in the province and writing a complex formula that goes through the data, looks for classes that exceed the pre-2002 caps on the number of students and special-needs kids and then creates new classes accordingly. The formula is so sophisticated that it even looks to minimize costs by searching for opportunities to create split classes, BCPSEA said.

But it's not an exact science. New special-needs designations have sprung up that didn't exist in 2002, which means BCPSEA had to make some judgment calls on how to classify students with those newer designations.

BCPSEA also used its spreadsheet to cost out the BCTF's proposal on class size and composition, which seeks even smaller class sizes than the pre-2002 language – and that's where the $2-billion number comes from. After the union softened its demands, BCPSEA concluded the ultimate cost to taxpayers would be $1.67-billion annually by the fifth year of the contract.

Mr. Iker calls the employer's numbers "fear mongering."