the money and getting a guide who sets up transportation. The experience without a guide didn't compare to the experience with one.
We found a guy who was willing to set up transportation, lunch and customize the tour based on exactly what we wanted to see. It was worth the money. He had lots of insights and details and was from a family who had lived in around Normandy during WWII. The transportation is important because we never had to worry about parking, and it gets really crowded. Then we used the rental car later that day after a lot of day-trippers had cleared out.
The next couple of days we visited other parts of Normandy and museums by car, seeing the things that we thought could be more easily covered without a guide. The other nice part was that during the tour, our guide gave us more tips to use during the second day.