Michael Anissimov has a picture of a progression from World War 2 soldier to a mech.
The hypothetical Mech has a hypothetical price tag of 500k-700K.
Military hummers cost $150K armored and $60-75k unarmored Of course they are not autonomous, although with the DARPA program for self driving cars..we could add yet (and tack on some price tag, but not necessarily that much with continually falling computer and electronic prices... if acquired in volume). All the weapons that the 5th gen system has could mounted on a self-driving humvee. Since the armor probably would probably not stand up to the weapons, probably better to go cheap on armor and go for unarmored and get two unarmored instead of one armored.
Actual vehicle being developed is the CMU crusher
The alternative to the "super alloy" of the Mech would be better ceramic armor or the new durable liquid metal (twice as strong as titanium) If you could get the exotic armor cheap enough and if it is worthwhile to make a tougher armored system instead of more cheaper systems.
The actual unmanned ground vehicles that is being developed is the CMU crusher
The Crusher program has cost $35 million so far Not clear how much any final per unit costs would be.
Current progress to autonomous robots is substantially because of better 3d LADAR
More on the DARPA robotic driving systems
Unmanned ground vehicles