The 100£ assumes nothing is in the car, not even a factory fitted stereo. Sorry, that is utter b***s.
For an average low-end car with a stereo sans GPS and sans 3G with some sort of stereo the incremental should be under 50£. This is roughly what it will cost to replace the stereo controls with Android or some embedded clone of Windows, add a limited SIM and basic GPS and connect the "active" indicator from the airbags control unit to a GPIO pin.
This cost drops to zero going upmarket. The ~50 is for todays equivalent of Peugeout 106 (or whatever the cheap model of the sole manufacturer obstinate to install Eu recommended safety features without a regulatory mandate).
The moment you go up from there the incremental cost to the stereo drops rapidly to zero as it is likely to be Android driven anyway, have traffic updates anyway and as you go in the upper half of the market have GPS anyway. This directive will simply accelerate this a bit.
The math is also broken - while you may save only 2500 lives you are also likely to reduce dramatically various costs across the medical systems by having a trauma team in place and in time even for less critical injuries. So you also have benefit for lives "improved", not just those saved.
The only people who will be bummering here are the mobile operators which have to deal with a few tens of millions of SIMs (including roaming) in continental Europe. However, once again - this cost goes to zero upmarket because the units will be using a service anyway.