Late Tuesday, the bipartisan Gang of Eight released a new version of its comprehensive immigration reform bill to the Senate. Now 867 pages — up from the original 844 — the bill, known as a substitute, is filled with corrections and rewordings of portions of the first bill. But it appears there are some substantive changes — including a whopping change that makes a key part of reform ten times more expensive than previously thought.

The bill establishes a “Comprehensive Immigration Reform Trust Fund” to cover the various costs of reform. It directs that when the bill is enacted, $6.5 billion will be transferred from the Treasury to the trust fund. And then the bill specifies money to be appropriated for the start-up costs of the process to legalize the estimated 11 million immigrants currently in the country illegally.

The original bill said this: “On the later of the date of the enactment of this Act or October 1, 2013, $100,000,000 is hereby appropriated from the general fund of the Treasury, to remain available until September 30, 2015, to the Department [of Homeland Security] to pay for one-time and startup costs necessary to implement this act.”

The substitute bill reads differently: “On the later of the date of the enactment of this Act or October 1, 2013, $1,000,000,000 is hereby appropriated from the general fund of the Treasury, to remain available until September 30, 2015, to the Department of Homeland Security and the Department of State to pay for one-time and startup costs necessary to implement this Act.”

The difference is striking: In a flash, $100 million in startup costs specified in the original bill became $1 billion in startup costs in the substitute bill. Why? In response to an inquiry, Gang of Eight aides say they are checking on the change. But others on Capitol Hill want answers quickly. “Why did the immigration reform bill’s cost increase by ten times in less than two weeks?” asks one top GOP Senate aide. “The authors are claiming that these are ‘technical’ fixes. Hard to argue that going from $100 million to $1 billion in startup costs is a ‘technical’ fix.”

UPDATE: After this item was posted, a Gang of Eight spokesman emailed to say that, “The initial $100 million number listed for startup was incorrect; $1 billion is needed to ramp up operations to handle 11 million applicants and other new visa programs. The money will be refunded to the Treasury from fines collected, so it is deficit neutral over the next few years.”