However, I still struggle to understand the art of estimating rehab costs. 

Cost of construction is expensive and about to get worse.

This can often be cost-prohibitive. 

Although I know most sec 8 landlords use it because of the perception of guaranteed rents.As a small home builder I really dont know how they are going to lower the cost of new construction housing unless the govmit subsidizes it somehow.. basically the price of home components are very sticky they only go up. with the xception of lumber that will go  up and down through out the year..

I own a duplex in Clermont, FL just outside of Orlando and initially wanted to split water meters between the two units, but the cost and amount of work were higher than expected.

You can also assess a dollar amount that you value the cost of these requests to be, and ask for these as a discount from the purchase price or a concession to you for closing costs. 

There were a lot of costs that we didn't expect like the plumbing had to be completely redone.

Real estate is not the stock market and it's called buy & hold and not buy & sell for a reason.I did sell a few properties about 10 years ago when I learned the hard way that going into cheaper neighborhoods was not the right strategy for me, so that was basically a strategy correction and it was necessary, but also costly both in terms of dollars and time.

The longer you hang out on the bridge the more it costs you.

I’m estimating renovation costs at around $150,000.