We are trying to come up with an attractive investment for and investor but since we have no way of knowing how much the property is worth, how can we form a fair deal structure?

If there aren't other assets or businesses in the LLC, you can "re-purpose" you can file another DBA as well.If there is any chance of prior liabilities, I would form a new LLC.

(Cash on cash roi/ monthly cashflow/ pro forma cap rate / etc)My properties at the moment are anywhere from 8% to 23% cash on cash returns, with cash flows ranging from $175 a month to $833 a month.

Then you can set up your plan as to how to hold title, form a TIC and rent them.

My GC walked the property today and he felt he couldn't do it for the pro forma amount.

Another options would be to form a "team" of sorts.  

I've started reading it and am on the Chapter about Discipline, where he advices to form a habit around the ONE Thing that will get you the most success. 

it can be very time consuming but if you can form a team to do it for you for reasonably cheap it is actually a very effective way of finding distressed properties in your own area.

We want to form a business organization together and he'll buy in to equal partners.

In true Texas form, a buyer is not required to disclose the purchase price to the government - meaning, the purchase price is not public record.