It might make sense to adapt different off the shelf aircraft for the purpose of transporting POTUS, depending on circumstances of the trip. Short hops, the smaller planes might be adequate. National Emergency, the stealth, for overseas travel a 747 or equivalent.
The question is one of whether the costs of maintaining the aircraft and crew requirements for different types of aircraft would be greater than the amount saved by using the smaller planes for shorter hops.
It would be a good political move after the Obamas' spending fortunes on vacations and travel, provided the numbers worked.
We do something similar with vehicles, with a small one for fair weather town runs (easier to park, more maneuverable), and larger vehicles for work/utility use and longer trips.