On average for sidewalk curb gutter etc. including utilities minimum $50k per lot.  

If you are flipping or in it for five years or less there is more risk (5.5 years is an averages economic cycle) If you are buy and hold your value could go down for a few years then pop up and climb, but during those five years rents will increase, you will cash flow, services/repairs become cheaper, and all the time you are financed in cheap money paying low interest collecting equity each month from the rent paying the mortgage.

https://www.courier-journal.com/story/news/local/2020/03/12/louisville-home-values-expected-rise-2020-property-assessment/4941202002/On average most homes are appreciating by $7,500 or $10,000 per year.

The average cost of a studio in NJ is roughly $1084.

From my own experience in managing my own properties it seems like what the expect of average joes with an hour or two of free time for a gig is a pretty tall order. 

What’s the annual average income on a property like that?

For example, if rehab cost $70K which is our average, he gets $4,900,,  PLEASE ADVISED, 

Timing the market for an average Joe is a terrible advice (aka sell low/buy high).

Realistic age is average age of the homes in the immediate area. 

The average, refinance closing costs range from 3 percent to 6 percent of your loan amount (again, depending on your location and your lender).