Andy I'm no expert on this but I think it would be fair to assume that the tilt-rotor is by far the more expensive (both to build and operate) out of the three.
As you know, the X3 uses 'caterpillar-style-braking' technology
for yaw control necessitating two thrust generators to achieve this and with that comes the expense of drive trains and gearboxes.
I personally believe that the X2's 'additional' rotor may turn-out to be the most cost-effective solution.
Cost aside .. there are certain landing/ground-handling (manoeuvering) restrictions which apply with both the tilt-rotor and X3. In the case of the former one needs to consider an increase in the landing area requirement as well as the implications of the vehicle's impressive downwash and with the latter (in its testbed configuration) the problem of side-mounted props just waiting to dice-up the craft's pax.