Agree on the cost to buy a vacant lot and build. To stay within their budget they would have to settle on 1,000 sq. ft. of living space. But even more important is the issue of being literally right next to the airport. In a couple of the views shown the roof of the airport terminal could be seen just below the lot. The realtor said they could mitigate this by building a sound proof structure. That means any outdoor living area would be subject to the airport noise. But most importantly in my view is that they were going to be opening a bicycle rental business presumably based in Phillipsburg. This would mean they would be subject to frequent traffic delays during their commute between the airport and downtown. The Point Blanche property would have been ideal for them in my opinion.
Update: After a second viewing I see that they currently have a bicycle rental business in Washington but are planning to open an electric car rental business in St Martin. In this case being near the airport is probably a good choice. The idea of an electric car rental business is another matter.