As I pointed out in a previous post, I agree that the sensors could be a liability. Of course as the cost of technology comes down, you could duplicate the sensors, giving you multiple backups in case of an attack.
A Black Knight would indeed be more expensive than an RPG-29, but so would a Bradley, Abrams, MRAP, etc. Soldiers aren't seen as expendable canon fodder anymore, so I'd rather see a million dollar Black Knight taken out than a Bradley full of dismounts. Even from an economic standpoint, it likely costs more to train and house the Bradley dismounts and crew as well as the vehicle itself than it would for a Black Knight.
So yes, unmanned combat vehicles will be more expensive than most insurgent weapons. I view them as an assistant to troops in the field rather than a replacement.