C-17

I just have some questions regarding C-17Globemaster

1) Why is it so so costly compared to other transporters :

— is it due to its weight lifting capacity.
— Due to its advanced Avionics
— is it stealthy in any way( I Dont think so, but there may be something i dont know)
— does it have any special capabilities, not available in other transporters
— Does it add any radically new capabilities to US Air lift capabilities, which were not available with C-141.