Vacant Lot Purchase Agreement

[Aaron Lozoya]

Hi community, quick question, 

I have a very hot lead for a vacant lot in the middle of San Antonio, Texas in the middle of a good neighborhood full of 20-50-year-old houses. This home burned down a number of years ago and the family has just let the vacant lot sit there. 

The home that was there was a 3bed 2 bath 1500sqft on a 9500sqft lot. 3/2 Comps in the last few months have sold for $180-200k. 

Question: What is a reasonable formula to use that would make this an investor-friendly deal. I understand that is a broad question being that details of the new construction details are hard to get into based on what comps look like in the area, but that is my dilemma. What is a vacant lot/ Comp price/ ROI formula that can work for a deal like this?

Side question, will the purchase agreement I have been using for single-family residential still work for a vacant lot like this one? 

Thanks in advance!