WHAT YEAR Grandis?
On the Plate bolted to your firewall what is the Model of the car? For Example LRPYR1D or LRUYR1D or something like that.
It uses up to 4, which one do you need? 14925, P, Q, S
mitsubishi grandis o2 sensor
those sensors are VERY expensive.... you may want to seek used one from local mitsu techs.
New you are looking at 22000 to 57000 depending on the exact one you need and the vehicle model. Cant give you an exact cost without your VIN and without you identifying which ones you need.