Google shows this:
French sim card with unlimited data in France
$7 a day for "unlimited" data in France, no voice, on the SFR network (picture shows SFR packaging and I chatted with them and they confirmed SFR).
But each card only last 3 days so you have to buy in increments of 3 days.
Still a better deal than most prepaid data in France, unless you have to have voice.
Tempted to try it as I have a trip coming up in 2 weeks and hadn't thought about bothering to get a French SIM because there are no good values.