1) Es gibt kein Leerzeichen dazwischen: €40
2) Man fügt ein Komma ein: €400,000
3) In diesem Fall benutzt man einen Punkt: €2.3 billion

Vielen Dank, wupper!

http://de.wikipedia.org/wiki/Schreibweise_von...

http://www.schreibbuero-schreibarbeiten.de/ht...

http://www.translationdirectory.com/article167.htm

When an amount is mentioned, place the accepted abbreviation before the figure, leaving a space between the abbreviation for a currency and the amount, unless the abbreviation includes a currency symbol

Belgian franc BEF 10.00
Euro EUR 10.00
U.S. dollar USD 10.00

http://www.economist.com/style-guide/currencies

re #3: You can use the three-letter abbreviation if you have to -- for example, in a context with a limited character set such as a telegram, or in a financial table or database that includes many world currencies whose symbols are not well known.

But in normal English text, such as essays, articles, and headlines, you should always use the symbol ($, £, €) for the euro, pound, and dollar.

You can use the three-letter abbreviation if you have to -- for example, in a context with a limited character set such as a telegram, or in a financial table or database that includes many world currencies whose symbols are not well known.

The three-letter abbreviations are standardised currency symbols for any financial text.
http://www.xe.com/currency/eur-euro


But in normal English text, such as essays, articles, and headlines, you should always use the symbol ($, £, €) for the euro, pound, and dollar.

The Economist always writes it out: euros, not €

Sorry, I meant if you need to use an abbreviation, it should be the usual one, i.e., the symbol, not the three-letter code. There's no need to avoid the symbol unless there's some reason your character set doesn't contain it.