I have AT&T. UNLIMITED data, not throttled, UNLIMITED calls/texts to USA, Canada, & Mexico. $10/day all other countries on planet Earth. $78/month. So figure $378 a month for 30 days spent in France.
My wife got her phone "unlocked" & bought a Vodafone Sim card when we got there & just had a new phone number for the country we visited. Sim card was $30-ish, a month of service was $30-40-ish. So, way cheaper than me, but she was limited to that single country, I wasn't. We relied on my data connection when we hopped over to Belgium.