What is the grammatical structure after the comma?

Even with the proposed budget cuts and new taxes and fees, the city's projected deficit for the next budget year is getting worse: administration officials announced that they believe the gap will be $3.7 billion, a billion dollars more than they had predicted just two months ago.

What is the grammatical structure after the comma (a billion dollars more than... )? I thought it was an appositive but "a billion dollars.." is not really restating another noun. Is it an adjective phrase?

Solution 1:

This could be an instance of ellipsis:

Merriam-Webster the omission of one or more words that are obviously understood but that must be supplied to make a construction grammatically complete

The complete version of the clause would have been:

the gap will be $3.7 billion, [which is] a billion dollars more than they had predicted just two months ago.

[which is] was [elided] omitted to make the sentence more compact.