: In 1999, the part of the population of the Paris metropolitan area living in Ile-de-France represented 98.996% of its total population. The same year, the part of the population of Ile-de-France living in the Paris metropolitan area represented 97.023% of its total population. In 1999, the demographical difference between Ile-de-France and the Paris metropolitan area was about 2%. There's nothing illegitimate in considering the Ile-de-France GDP as an estimation of the Paris metropolitan area. 
 : Actually, the conversion from Euros to US dollars leads to a much stronger approximation of that figure, knowing that both currencies have fluctuated in a margin of 10% in the last 12 months. 
 : You see, ThePromenader, your behaviour is in here exactly the one I can't stand anymore. It consists in finding a little trick to make disappear a valuable information, for a reason I fail to understand. The GDP of Ile-de-France is definitly a legitimate estimation of the Paris metropolitan area which is perfectly legitimate to mention in the article about the economy of Paris.  . 
 ::Normally, I would agree, however.... WP:NOR. You need to have a precise source for the data, estimations from data on other regions/populations is considered original research.   
 No, no and no. If there are no GDP figures for any "metropolitan area", one cannot cross-calculate numbers on his own to make the term fit and still call it fact - especially through calculations based on data so non-sequitur as land area and population - GDP is calculated on the number of businesses, and the revenues thereof, within a given territory, not land and population. If one insists on using the aire urbaine as a base for GDP, he will have to do the data gathering in the said area himself, as no fiscal data has ever been taken only in the wedge of extra-IDF territory covered by the AU.  

 This sort of maniplation is the very example of Original Research. Because it is in error, is Original Research and unverifiable, it is of no factual nor informative use to anyone. Certainly not to Wiki.