SBAS True Costs
The real actual cost figures for SBAS are actually worse than $4B for the US alone. The numbers you're typically seeing from ANSPs and vendors (who usually have a stake in the outcome) are NOT the fully allocated costs. Further some locations are considering SBAS for "Political" reasons, and not technical reasons, or they are being talked into adopting it by vendors selling "stuff", or they are simply poorly informed as to the true options, better and less expensive alternatives, lack of real benefits, and hidden unnecessary costs. Galileo will eventually happen. But even before that, with 30+ GPSs and SA off, there is no longer any need for SBAS in the US or anywhere. That's why airlines have been safely and successfully flying both RNP .1, and some GLS too, totally without SBAS, for well over a decade now.
Now, if we had more rational criteria by authorities, and not the present over-specified and unnecessarily complex present criteria (e.g., exceedingly poorly framed AC20-138, 90-101, etc), ... then even $2500 total cost UAVs and other vehicles, from LSAs and low end GA up to retrofit jets, could even be using it routinely and economically too, with accuracy, integrity, and availability, ...all with simple low cost GNSS Kalman filtered sensors, in FMS equivalent nav and display systems.
SBAS is now an unnecessary system whose time has come, ...and gone.