Help finding an offer price

[Sean Smith]

Newer investor here...planning on making an offer to my immediate next door neighbor in a commercial property. It used to be 1 large build and lot, but was split up years ago. The construction materials (it's an old building) are the same and we even share a load bearing wall. 

I know roughly what the owner's numbers are already based off of mine. They should be free and clear at this point. 

Rent roll: $2800/mo
Taxes: 200-250/mo
Insurance: 125/mo

What other numbers do I need? 

I haven't been inside all of their units but they look decent from the outside. Can someone show me how to work up an initial offer to present them? Thank you.