In agreeance with @Sheryl Sitman Philly as a whole is extremely block by block and your ARV values can change dramatically within a block radius.

And how do you get a good number to calculate with if the costs changes dramatically throughout the year?

For stock market investors, they think that there will be a market crash.I don't think that the crash we saw in 2008 will happen that dramatically. 

Now four years later in the inspection report from my buyer a more dramatic statement is made: "Polyethylene Plumbing Supply Lines have been known to to be randomly defective and can rapture without notice" and the buyer wants me to replace the Polybutylene Pipes in the condo.

This will dramatically lower both the interest you pay overtime, but pay the loan down much faster and keep the payment down so the property cash flows.

Two big factors I'm seeing are dramatic rent increases which will bail out some investors with loose underwriting criteria, but at the same time expenses are also increasing like crazy (especially property insurance, taxes, cost of materials and labor, appliances, etc.).

Their chances of ever getting into public housing and Section 8 also take a dramatic nosedive.This is useful information to bring up when discussing rent collection issues with deadbeat tenants.

This is a C class neighbourhood in Edmonton, Alberta and after the condo fees increased dramatically, it no lomger meets my investing criteria. 

The majority of the Island is zoned for 90 day min rental which dramatically impacts the price.

I've seen a lot of rehabbers cut major corners and try to leave unsuspecting buyers holding the bag.Not only that but they rarely pull permits, and if a future buyer ever checks for permit status and finds none, the property value goes down, I've seen sometimes dramatically like 50% per SF for non-permitted living space.....