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?

About 20% of the pesticide will be absorbed by the mouse. This is because the direct effects of the pesticide were already absorbed by the grasshopper and processed within its body system, also probably leading to excretion in different sorts. Therefore the effects of the pesticide is lesser compared to the mice. 

That would be a working theory.