We've caught the no.50 bus to Provins, we went on a Saturday and I think it was every hour on the half-hour but as this was back in 2008 the times could have changed. I know we'd looked online at the timetable but went and checked out the one at the bus stop the day before just to make sure of the time. If I remember correctly the fare was 5 return each and it took an hour to get there. The tourist info centre is right by where you get off the bus and the staff were very helpful. Carrie's right that you don't need to spend money on a guided tour, we went every where on our own steam. I remember we hired audio guides for one of the museums and watched an equestrian show that was really good even though we couldn't understand a word (!)
It really is a lovely day out, there's lots of restaurants there but we bought some picnic items and found a spot to enjoy that.