Final answer:
The external cost of the dump is $50,000.
Step-by-step explanation:
The external cost of the toxic waste dump can be determined by calculating the difference in average home prices between the neighborhood near the dump and the other neighborhood. In this case, the difference is $90,000 - $40,000 = $50,000.
Therefore, the external cost of the dump is $50,000.
This means that the presence of the toxic waste dump reduces the value of the homes in the neighborhood near it by $50,000 compared to the other neighborhood.