Four of our family members took this trip in April, 2012. They were very disappointed that the Pont du Gard optional was not available, but managed to arrange to see it anyway.
The day before you plan to go, your program director can arrange for a taxi to pick you up at the ship to take you there. The journey takes about forty minutes. You have to have the taxi wait for the hour it takes to visit the site, for it is out by itself and is fairly isolated (from taxi stands, anyway.) Then it takes another forty minutes to travel back to the ship. They really enjoyed their visit there, and have enthusiastically talked about it a lot.
The cost of this, for four people, was $95. The taxi fare was $75, and the cost to have the taxi park and wait was $20. (I'm sure they probably tipped the driver, too.) Still, for a cost of $25-$30 each, it was quite reasonable.
I just called one of my relatives to make sure I had all this information correct, and was asked to tell you that this is something WELL WORTH going to see. It has stood for over 2,000 years and still looks good; the Romans really knew how to build!