Even though all of the grasshoppers weren't killed, they all were exposed to the insecticide, so when the mice eat them, they are also ingesting pesticide. how much pesticide will each mouse take in compared to each grasshopper

This is a process called bio-accumulation, where the concentrations of chemicals, eg insecticides increases as you go up the food chain. 

Mice are larger than grasshoppers so they require more energy to survive, the mice consume lots of grasshoppers and so get a lot of insecticide in their system, more so than grasshoppers.