The S&P unit that came up with the figure is the valuation and risk strategies office, which is separate from the ratings division, The Hill newspaper reported Saturday.
Nonetheless, in putting a dollar figure on a downgrade, managing director Michael Thompson said, "What we're trying to do is give people in the marketplace a prism … to what the effect of (is) of something they've never really bent their minds to."
"People never really went down this analysis because they just thought it was an impossibility," he said.
For years, figuring out the cost of a downgrade in the government's credit rating was thought to have no practical purpose, as it was, in theory, never going to happen.
However, with Republicans threatening to vote no on raising the U.S. debt ceiling, a downgrade is suddenly not only possible, it is lurking nearby.
The Treasury Department has said the deadline for raising the debt ceiling is Aug. 2. After that, the government would have to stop paying about a third of its bills, the department has said.
This week, Republicans said they were pulling out of bipartisan negotiations on budget cuts and the debt ceiling to protest Democrats who have advocated for tax hikes.
The impossibility of the U.S. credit rating slipping? "That's starting to erode a little bit," Thompson said.
Moody's Investors Service has warned the pristine U.S. rating was in jeopardy if an agreement on the budget was not reached soon. Fitch Ratings has said the U.S. outlook could turn from stable to negative if an agreement was not reached by August.