When are GPS Airspeed meters and Altimeters going to be introduced if they not already in use? Are they more accurate than systems in use now and? How much would it cost to install them to older aircraft?
In general, even very low cost GPS units can provide very very accurate velocity vectors.
Altimetry is not quite as good, and is different in that it is referenced to a geoid, rather than a barometric pressure.
Similarly, airspeed is referenced to the ambient air, and while a GPS is a very accurate velocity vector source, there is no easy way to know the ambient air velocity.
So in short, by necessity to maintain safe flight, airspeed utilizes relative wind on an aircraft, and there is not an easy way to determine that with GPS alone. And altimeter settings by convention, are based upon ambient air. However, by utilizing a geoid model reasonably accurate absolute altitudes or MSL altitudes could be determined. Cost? As much as an old cellphone and some software. Pretty cheap.