A word to the wise from someone who has been there.
In 1995 I took my family to Europe and stopped at Pont du Gard. We rented a motor home and we stayed at a campground. Somehow, in looking for the campground, I wound up on the wrong side of the river from the campground. Not knowing that it was prohibited, I drove the motor home OVER the river on the lower level of the aqueduct. It was only when I arrived on the campground side that I realized that this was a big NO-NO. Much yelling ensued, and my kids shrunk down in their seats as I profusely apologized to the angry Frenchman who was yelling.
Long story short, the day got better after that, but there were no fees to visit the monument back then.
So, good luck with the ticket purchase and remember, DON'T DRIVE OVER the river on the aqueduct.