Having checked French prepaid data tariffs, I came to the conclusion Vodafone Germany's WebSessions (€ 14.95 for 24h internet access, capped by 50 MB) is the best choice for real internet and a French Orange SIM with "Internet Max" option (€ 9/month for handset-based access only!, capped by 500 MB,
http://prepaid-wireless-internet-acc...+Orange?t=anon) is the cheapest option, if you access the web from your handset only.
Quite surprising, that there's no really attractive offer for mobile internet in a touristic country like France.
French Orange prepaid SIMs (called "mobicarte") are sold for approx. € 15 at
www.ebay.fr and German Vodafone prepaid SIMs (called "Call'ya")
starting from € 1 at eBay.de