Plus the cost of subdividing , and all that goes with it are for deep pockets . 

Over estimate the costs and not need them?

On your tax return you would report the proceeds, subtract costs and proceeds paid to seller, then the remaining profit would be your flat fee.

He's changing the terms of the agreement, and you can choose to accept it or reject it.I don't think it's fair to pass this cost onto the tenant just because you are unwilling to stand up to the contractor.

Am I more likely to be able to add $50k or $100k of value above cost in a high Cost RE market or a low cost RE market?  

Utilities cost $150 per month.

I'm looking at monthly cost to seller vs. anticipated rental income, purchase price relevant to the ARV of the area and interest rate, balloon, etc.

Low cost, possibly yes, but also possibly very time consuming and that has a cost.  

Here is an example of what it costs you to avoid professional advice.

All that being said, it might not be worth the cost of putting in parking, just be aware on the front end that you’re reducing your end buyer/rental pool by not having it.