SO, at least the BP calculator has little suggestion bubbles to give you a hint if you truly have no idea, otherwise, at least start ball-parking the water bill, for example, based on your current water bill at your residence.

This data will tell you where the market was and is which is your best crystal ball for where it is going.

If I’m luckily and low ball I could get $100-$200/month.

They are only meant to get you in the ballpark and it sounds lie the ball park is $2500 - $3000.

In my experience I have had some builders/manufacturers push back against this process but if they want the business then they will typically play ball. 

There's a construction software called Precise Builder that does exactly what you are looking for.

And it does not work every single time, life throws you curve balls.

Step 1 in marketing is getting as MANY eye-balls as possible to see that listing.

Usually after walking through the property he is able to give me a good ball park figure of what the renovation should cost barring unforeseen issues.

I know or know of people in your same position (executive level tech experience + capital) who have done well in this business, and also ones that have imploded in a spectacular ball of flames.