Randonnee: you pay $15 or so to ride 200 to 600 km. That's 7.5 cents to 2.5 cents per kilometer. They give you a route sheet, but the roads are not marked. You must obey all the red lights and stop signs. There might be food at the controls; if you're lucky there might be a volunteer who will stamp your card; otherwise, you have to ask somebody behind a counter to sign and timestamp your card. If you lose that card, you are SOL. If you get a mechanical, you must fix it yourself. Nobody cares how fast you did the ride, just whether or not you did it within the time limit.
Gran Fondo: you pay $200 ($500 if you are a VIP) to ride 160 km. Thats $1.25 to $3.13 per kilometer. You might get a route sheet, but you don't need it because the roads are marked, and besides there are so many other riders that you just follow them. Or if you paid the VIP rate, you're in the front group with some celebrity pro bike racer and a lead car in front of you. There is usually pretty good food at the rest stops, but you don't have to stop. They have marshalls or police at all the controlled intersections, so you don't even slow down for stop signs or red lights. They give you a timing chip to put on your bike, and at the longest, steepest hill, they time you to see who's the fastest up the climb. If you get a mechanical, just wait for the roving race service vehicle. Everybody compares their finish time at the end.
Luis