As a sensor gets larger, the cost increase is not linear. Right now, a FF sensor is very expensive to make, a 36 X 36 might cost 10 - 20X the price. The number of pixels on it are not the factor, its the area.
Then, the lenses will start at $3500 and go up, - way up.
Heres a somple 35-90mm f/4.5-5.6 MF zoom, imagine the price of a f/2.8.
http://www.bhphotovideo.com/c/product/613798-REG/Hasselblad_3023590_35_90mm_f_4_5_6_HCD_Aspherical.html