There are few realtor I talked to say, we charge this amount and said, I wont get a discount on houses we look at.

Depending on how much material or items you need there's always people selling excess material and items they ended up not using or needing, typically at a discount.

You may have heard Lake Michigan has risen six feet in the last ten years, then it dropped three feet recently, yet somehow this property appears unaffected: https://wcrz.com/the-worlds-largest-log-cabin-is-in-michigan...People love to point out rising levels but discount levels when they drop.

I do discount useful life and increase costs to be on safe side. 

I would try to get him to take possession earlier for a discount so at least you have utilities and etc out of your name and some money coming in earlier even if he doesn't fully move in earlier.

Apts complexes consisting of 100s or units can afford on site property manager, assistant managers, maintenance staff, volume discounts, etc.  

Can the deal be discounted if the GPs were investing their own $ too?

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Most buyers, but not all, are looking for discounts to "market value." 

Buy the loans at a discount, and strike a deal with the owner dealing from a position of strength.