There are three common ways, one is to do it strictly on an hourly bases, another is lump sum (usually based on their in-house calculations on who needs to work on it and how many hours they think it will take to complete) and lastly they could do it as a percentage of construction cost (usually in the 5-10% range, could be more).  

Hey JosephGreat job pulling median rent for both 4/2 and 3/1 to see what the range of rental prices are in the area.  

Here is my two cents from what you've provided; In general, your ranges for rehab, acquisition and ARV are way too broad- this could either be a great deal or a terrible deal, depending on where you landed within those ranges.

Applying for a primary residence HELOC on a duplex, in the range of $110-150k to perform some renovations and use for down payment on new duplex purchase.

Once in a rare while someone will say a number that is actually below the range of what I was prepared to offer.I will evaluate every lead as a flip, BRRRR or an assignment to give myself options.

The realtor that wanted to introduce me to one wanted a finders fee of somewhere in the 12k range.

These were largely sourced pre-CV19, so eat the fish and spit out the bones.The DTI percentage range varies by lender, and is less than what you will find for an owner occupied property, due to lender risk.

Obviously that land and common area has expenses (ranging from insurance and taxes to maintenance) and yet can’t bulldoze the building on their property.

Similarly with the rent ranges - make sure you get opinions from property managers, online tools (rentometer, etc...) to see if your rental estimates are current.

Do other comparable units in that area/price range provide them?