Learn English – Article before adjective+noun: Why is the indefinite article used in “a staggering 2.5 million”

I think that the discussion will involve a tangent about word order for adjectives. English has a customary word order for adjectives, discussed elsewhere on the Stack Exchange. All we need to say here is that idiom can require a certain order for adjectives, depending on what one is trying to say.

I believe that all of the following numbered sentences are correct grammatically, and native speakers of English would generally understand them as I explain them. Let’s compare:

1. The party included four horrifying people.
2. The party included a horrifying four people.

Sentence 1 means that however many people were at the party, four of them were horrifying: they were racists, or they smelled bad, or something of that nature.

Sentence 2 means that the total number of people at the party was four, and that is a horrifying number. It does not comment on the guests themselves; the guests may have been perfectly nice, for all we know.

It seems to me that idiomatically, the construction you’re asking about is the way we express a judgment about an amount. As a matter of idiom (rather than clear logic) we express that judgment by leaving the description of quantity next to the noun, and putting the judgment in front with an article.

3. The concert lasted for 25 disappointing minutes.
4. The concert lasted for a disappointing 25 minutes.

Sentence 3 means that each minute of those 25 minutes was disappointing. The music proceeded for one minute, and the audience thought “That was lousy.” Then it proceeded for a second minute, and the audience thought “That was lousy too.”

Sentence 4 does not mean that. The word “disappointing” does not describe the minutes. It describes the idea of 25 minutes. Each minute may have been great, with the audience hoping for two more hours. But then it ended, and the audience said “Hey, that was only 25 minutes!”

They were 25 great minutes, but still a disappointing 25 minutes.

So, to bring it home:

5. The car costs 2.5 million staggering dollars.
6. The car costs a staggering 2.5 million dollars.

Sentence 5 means that each dollar was staggering. The buyer plunked down the first dollar, and the seller said "Whoah! That one's green. Where'd you get that?" The buyer plunked down the second one: "Oh man, that one's green too!" And so on, 2,499,998 more times. That would be grammatical, but it might not be what's intended here.

Sentence 6 means that the amount is what's staggering, not (perhaps) each dollar considered separately.