You look up formula for the cap rate and then you can see asking price based on a 6,7,8,9,10 cap etc.
You have to go by income to get the cap.If there is no income then you evaluate differently. Once the seller sees what they would get for a sale sprice they will want ti fully leased before they sell.
If there is no rent it is just a structure someone is buying with non-productive debt (no cash flow) and they will have to make it productive equaling more risk and LESS money offered to the seller.
Good luck.