Treasurer Jim Chalmers selectively quoted a cost estimate of the Coalition's small business lunch tax break, despite its caution that the figures were of "low reliability". Mr Chalmers has used the costing to claim the Coalition proposal, which would allow small businesses to deduct up to $20,000 a year to buy meals and entertainment for employees or clients, would cost $1.6 billion a year or as much as $10 billion if every eligible business claimed the full amount. But an email chain tabled in the Senate at the Coalition's request has revealed that Treasury officials said the policy would cost only $500 million a year if restricted to meals only. That lower figure was omitted by Mr Chalmers when the costing was released, even though Shadow Treasurer Angus Taylor had already clarified that the Coalition's policy would only apply to meals. The use of Treasury to produce this figure aggrieved Mr Taylor, who said it was an inappropriate use of the public service and wrote to Treasury secretary Steven Kennedy to complain. Mr Kennedy, who will appear at Senate estimates on Wednesday morning, agreed it would be inappropriate to cost a policy "specified" as the opposition's, but said Mr Chalmers' request had not mentioned the opposition. An email chain tabled in the Senate at the Coalition's request has revealed that, while this is true, Treasury officials were aware their task had a political context. While this was rarely put into writing in the lengthy back-and-forth between the officials that continued after the preparation the costing, one of them quoted Mr Taylor's own words in the process of trying to figure out how to interpret the policy. "Ultimately this is one where the devil really would be in the detail of any policy/law change," assistant secretary Richard Maher told deputy secretary Diane Brown, pointing out Mr Taylor had left ambiguity about how the deduction for "meal and entertainment expenses" would be defined. That confusion was one of many reasons Treasury's final costing urged caution. At the request of Mr Chalmers' office, it provided two alternative costs — one restricted to meals alone, and the other including recreation such as golf trips and corporate boxes under a broader ATO definition of entertainment. The second option gives a significantly larger result, of $1.6 billion a year compared to $500 million, using data from a comparable deduction in the United States. But even though Mr Taylor clarified in the days after his announcement that the policy did not include golf trips or non-food entertainment, Mr Chalmers would go on to quote only the higher number. He has also emphasised the even larger figure of $10 billion, which Treasury had calculated only to indicate the largest possible figure if every single eligible business claimed the full deduction, likely by claiming outside the rules. That figure is more than 60 times larger than the estimate the Coalition says it received from the independent Parliamentary Budget Office, although it has not published this costing. "Please note that there is limited data available, and therefore this costing is of low reliability," assistant secretary Yiyong Cai told the Treasurer's office. "We know this policy costs billions of dollars but we still don't know what Peter Dutton would cut to pay for it," Mr Chalmers said upon the release of the costing. The practice of costing opposition policies has a long precedent, including a series of costings of Labor's 2019 election platform under then-treasurer Scott Morrison.