If I was to try and calculate the amount of potential investment, I would start from the Brown University study pointing out that every urban freeway leads to a population loss of about 17% of the total city's population, and work backwards from there. Without the freeway, theoretically, that 17% would want to return (obviously not the same people) and would need a place to live. In a city of 1 million people that is 170,000 people(!).
Optimistically assuming that a freeway removal project could capture all of that population (re)growth, would immediately reposition large areas of the city. For example, if RL Thornton (30) was removed from between Stemmons Highway (35/Mixmaster) and loop 12, that immediately increases the quality of life and therefore value of 1) downtown, 2) cedars, 3) Deep Ellum, 4) Fair Park, 5) and much of the rest of East and South Dallas.
For the sake of round numbers, let's say those 170,000 move in at an average house/construction cost of $100,000. That equates immediately to $17 billion (once again !!). Not to mention the amount of taxes those people would pay to the City of Dallas, imbuing its coffers and ability to maintain itself.
Those 170,000 would also now be in more location-efficient housing, meaning less transportation costs for them, likely more transit riders to help DART's numbers, would need local retail services so there would need to be auxiliary commercial investment (ideally small, local business and the multiplier effect therein), and would likely not have to spend so much on gasoline/cars which can mean upwards of $7,000 per year in their pocket that they can choose to do whatever they please with it: invest it, save it, or do the really American thing and blow it all in Vegas.
The point of all this, is that the economics of cities get really complicated to the point of being nigh incalculable, if you want to do them right. So there is a certain amount of faith involved, particularly in rejecting the numbers that are used, because those tend to be so oversimplified as to be useless. We use them anyway and as statistics are often mere abstractions, we end up with a similar abstraction of a city.