Sam, you are correct 16 euro each for a 40 minute light show is a bit expensive. My wife and I talked it over and decided not to purchase tickets in advance (70 of the 200 tickets are supposed to be available at the box office). If we were to buy in advance the best night to schedule would be our last night in Rome. Do we want to just stroll and enjoy Rome, use the money to allow for a splurge dinner, or go to the show? Now, that will not be our last night in Italy, we still would have another 3 weeks to go.
So for now this show has gone to the category of "possibilities" on our itinerary.