Think 250 is far too high...just looked at one of our neighbours, Allana Potash. In their feasibility study (MOP), they have OPEX of 98 Dollar.
Here is what I expect for STB:
Production rate module 1 = 350kt
Capex = 350 - 390 Mio Dollar
Opex = 70 - 80 Dollar
Price SOP = 650 - 700 Dollar
GLTA