The price of extending Republicans’ tax cuts will surge by hundreds of billions of dollars if lawmakers dawdle, new government figures show.

The Treasury Department released numbers Friday showing that rolling over all of the soon-to-lapse provisions, as well as undoing other reductions in business benefits that were triggered by the 2017 law, would cost $5.5 trillion — substantially more than the $4 trillion the Congressional Budget Office has projected.

There’s a number of reasons why Treasury’s price tag is much higher but a big one is that its analysis begins in 2026, not 2025, like CBO’s.

That may not sound like a big deal, but when budget analysts estimate the price of legislation, they look at the anticipated expenses over a decade, and it makes a huge difference when the cost-counting clock starts.

Right now, the government is in its 2025 fiscal year, and so analysts look at years 2025 through 2034 — and expenses in 2025 are minimal because Republicans’ tax cuts are still on the books. About 40 provisions are due to expire at the end of the year.

But when the new fiscal year begins Oct. 1, analysts will begin tallying up costs between 2026 and 2035, as Treasury happened to do in its in analysis. That means the very cheap year of 2025 will fall out of the picture, and the very expensive year of 2035 — when costs are expected to run $600 billion — will suddenly be included.

House Speaker Mike Johnson (R-La.) has said he wants to get a bill including the tax provisions on President-elect Donald Trump’s desk by the end of April — an ambitious timeline given the size of the package and Congress’ tendency to dawdle. Some tax vets now expect lawmakers to take until their August recess to wrap up their plans, if not longer.

Republicans are off to a slow start, with lawmakers bogged down in an extended and so far unresolved debate over whether to tackle immigration first and put off taxes until later or combine them into a single bill.

The House and Senate intend to go their own ways, with competing approaches, which amounts to postponing critical decisions, like how much to spend on a tax bill, that have to be resolved before they can use the “reconciliation” procedure they’re depending on to get their plans through the balky Senate.

Treasury wasn’t trying to warn Republicans of the cost of delay; it was examining the cost and the biggest winners of renewing the provisions. Not surprisingly, the analysis says the super-rich would see the biggest benefits, with the top 0.1 percent reaping a 4.2 percent increase in their after-tax incomes compared to the roughly 1.2 percent increase for those in the middle of the income spectrum.

CBO is expected to release next week its own revised estimates of the cost of extending the provisions.

Last year, it put the price at $4 trillion or $4.6 trillion, including increased debt service.

It’s not like Congress to work far ahead of schedule, but had lawmakers taken up the extension of the Tax Cuts and Jobs Act in 2023, the price would have been easier to swallow. CBO pegged the cost then at a comparatively paltry $3 trillion, or $3.45 trillion including interest expense — because its estimate then included not just one but two cheap years, 2024 and 2025.