.The minitel was free (we had to choose between having a minitel or having a paper annuary). The communication on 3615 was expensive (9.15€ per hour). On 3614 or 3621, it was around 3.05€ per hour (sometimes half).
A bit expensive in 1992. I say this as a Spaniard, OFC the costs of living in France was higher than ours, now are pretty even.
EDIT: per hour, about 5C per minute, or 2.5C per minute. It's like a local call, then.