How to estimate ARV and cost of repairs

[Qais Attarwala]

I wanted to get guidelines for estimating how much certain types of repairs would cost. How do investors look at the state of a property and judge that it will probably cost x amount to renovate? Also if the ARV = Market Value + Value Added from Reno, how do you estimate what the ARV would be? Is it just the market value of sales comps in the area? But then how do you factor in the amount of value that you added to the property during renovation?