The cheapest option out of the lot and reusable. As I've said, the plan is for these things to sit at about 70,000 ft in the polar region until needed and then sent on their merry way. The only limiting factor is the number of times the battery can be recharged. Commercially they will be replacing satellites. Boeing's biggest breakthrough has been to come up with a lightweight design that is able to cope with the wind speeds at these altitudes.Except drones are vulnerable to many things as well, and are not cheap items to procure.