Sunday's print edition of the New York Times carried this headline on page 2 of Week in Review:
Awaiting Handshakes Worth a $100 Billion
Read it aloud. Now read it the way it appears in the online edition, where some sharp-eyed copy editor evidently made a fix:
Awaiting Handshakes Worth $100 Billion
The second headline is correct; the first one is embarrassing. You don't need the indefinite article "a" in front of the dollar sign, because "$100 billion" is already pronounced "a hundred billion" (or "one hundred billion"). You wouldn't say "a one hundred billion dollars," would you?
The Times is hardly the sole perpetrator of this particular malfeasance, just the most prominent one I've caught to date. The erroneous construction shows up chronically in blogs, press releases, and corporate web sites. You see it only when the monetary figure starts with the numeral 1; people know better than to write "worth a $250 billion." (However, I have seen "worth a $100 thousand." That figure should be expressed "$100,000," sans "a.")
Just to clarify: "A" is required in front of "$100n" (thousand, million, billion, etc.) when the dollar amount is modifying a noun and "dollar" is singular: "A $100 million gift" ("a one hundred million dollar gift"); "a $100 billion question." When it's part of a noun phrase and "dollars" is plural, drop the "a."
And, yes, the rule holds with euros, pounds sterling, yen, and every other unit of currency.