Cost to Cure and Curable defect assessments

[Brandon Gamblin]

I want to elaborate on a term I ran across in my real estate dictionary. 
"Cost to cure - The amount of money it will take to remedy a cause of depreciation. A curable defect is one for which the cost to cure is less than the value added by correction."

This is the example it gave: "The cost to cure an aging, leaking roof is $8,000. It is considered curable because the house will be worth $10,000 more tha it is worth now".

My question is how do we know or how can we tell beforehand how much value a single repair puts on the value of the home? How is it determined how much a property defect takes off the value? For a hypothetical example, If a house has an "as is" value $50,000, and needs roof repairs, how much value does the repairs add to the value of the home? What factors influence the amount added on to the "as is" value of the home?