Now, the next thing to do is to make a price drop / rent increase estimate that would be required to bring this $991 dollar difference into the historical norm of $170.

To make things simple, though, you should assume no rent increases for the next few years since many flippers/home sellers are putting houses up for rent while most apartments have vacancies.

Add in a recession, and you could also have rents dropping at the same time real estate drops in price, as well.

As a sidenote, CalculatedRisk has a nice price to rent ratio chart for San Diego:

[Link]

With rents unchanged, we are looking at a drop from 1.6 to 0.8 (recession), which would correspond to a 50% drop from peak prices..