I'm struggling to find a useful cite on the web that covers this, but from what I recall from my college courses on the lost generation, the impetus for Americans emigrating to Paris during the 1920s was less prohibition and more the fact that France underwent a peroid of steep inflation in the aftermath of WWI, meaning that anybody being paid in dollars could live much, much more cheaply in Paris than they could in say, New York. So if you were a writer or artist selling your work in America, you could make enough money from, say freelancing for American newspapers (as Hemmingway did) to live pretty comfortably in France and still have enough time to concentrate on your creative work, which would have been much more difficult back in the States. (Plus have time off for the occassional ski weekend in the Alps or fishing and bullfighting in Spain, etc.) The relative openess/relaxation of European culture about stuff like booze and sex, plus the great art and music scenes in Paris were all additional enticements, for sure, but the main thing was that it was a helluva lot easier on the wallet to be a starving artist in Paris than it was back home.

Of course, the dollar going so far in Europe at the time also meant you got a lot of rich Americans spending a lot of time abroad during that period as well, buying art, marrying nobels who had their fortunes wrecked during the war, and generally partying it up....you see more of that in Fitzgerald's stuff, since he was a best selling author from a young age and used to run with a fast crowd.