Money as a compound adjective

There are times I look at the answers on AP's Ask the Editor section and just kind of wonder ...

Here's a recent exchange:

Q. 10,000-euro project or 10,000 euro project? from Evanston, IL
A. 10,000 euros project.

OK, for years and years and years, the standard American English formation has been that when you put the money amount in front of what you are modifying you drop the "s" and add a hyphen. "A cigar costing 5 cents" becomes "a 5-cent cigar."

So, pray tell, why would this not be a "10,000-euro project"? I'm sorry, but that's what I'm teaching my editing students until someone shows me conclusively why it should be different.

(One recent form of this has been in regards to South Carolina's cigarette tax, which at least one paper refers to as a 50-cents-a-pack raise. I can almost see that one's logic, the "cents" modifying the "a pack," though I would still contend the idiomatically accepted form is 50-cent-a-pack raise.)