Your comparison is valid for the scenario 'whats the difference in price for x people to make this trip by car or plane'. His comparison was 'how does the cost

Your comparison is valid for the scenario 'whats the difference in price for x people to make this trip by car or plane'. His comparison was 'how does the cost of the trip change if I go from one to two people for either method of travel'. Assuming he and his wife operate as a single financial unit, then the cost of the trip by car does not change when they both travel somewhere instead of one person travelling, and you have to make that assumption or his mention of 2 train tickets being more expensive makes no sense.