Evaluate property w/multi family unit development upside

[Jay Charles]

A seller called me this morning about a 3/2, 1300sf living space on a 13000sf Lot property surrounded by multi family units to the front, its back and almost developed mulit units on it's sides. It's pretty much stuck right[b] in the middle. I looked in the county property tax record the land is appraised at $47,000 building $18,000. This property looks to have a multi family unit upside and potentially be worth a ton of money if developed right. Seller wants $200k for it. Seller said he's done some research and his property could be worth about $1,000,000. How do I evaluate the property if looking to wholesale it to a developer? How do determine what to offer to seller?