@Eric Thornton as has been mentioned by @Matt Engle, @Theo Hicks, @Ellie Perlman, @Omar Khan, @Jeff Kehl, and @Henri Meli, there are subtle differences between asset classes in CRE that need to be considered when underwriting a potential deal.
But the basics of Income - Expenses = NOI divided by the current market CAP rate will give you a ball park estimate of what it could be worth, all things being equal. Of course all things are not equal and you'll fine tune your opinion, or rather you will verify your opinion during the due diligence process.
For instance, if we analyzed @Michaela G.'s small Georgia retail property and determined that after some minor exterior rehab she would be able to negotiate NNN leases, meaning the tenant pays all expenses, with (4) local service retail business at $1,400, $1,300, $1,200, and $1,100 per month each, making a total monthly income of $5,000 or $60,000 per year net to landlord. We would basically have the Income and Expense totals we need to do a quickie, back of the napkin analysis.
If we then determined the local market CAP rate for small retail properties was 8% we would estimate the rough value of her property to be $750,000.00. ($60k NOI / .08 CAP = $750k) Of course there are almost always some expenses that need to be included in the analysis even on NNN properties, but that's easy enough to subtract before NOI.
However, the market CAP for small retail properties in Georgia may currently be 6% in which case the rough estimate of value would be $1,000,000.00. ($60k NOI / .06 CAP = $1m)
And before @Michaela G. can pay $750k or $1m for that vacant property she needs to determine exactly what it will cost to fix it up enough to attract the (4) tenants. She also needs to determine what the anticipated starting rent will be for each suite to make sure there is enough potential future income to justify the investment. And she needs to determine if there are at least (4) potential tenants available in her area that will want to operate future business at her property, and if those (4) business will work good together to attract the right kind of customer for each other's benefit.
She will have already determined the building had great visibility from the street; that there was plenty of parking for both tenants and their anticipated customers; that ingress and egress onto the property would be easy and safe; and that future customers could easily find the building when they're ready to come spend some of their hard earned dollars.
The good news about small retail properties is the tenant improvements (TI's) are usually minimal and often times the tenant can perform the work themselves. @Michaela G. will focus most of her initial rehab investment on the exterior to make the property becomes a very attractive place where local business will want to re-locate and can see themselves succeeding there long term, often (10) years or longer. She will of course set aside $3-$7 per SF for carpet and paint on the interior but anything more than that will be at the cost of the tenant or at least amortized over the term of the lease if she wants to lend them the funds for their TI's. She will also set aside 5%-8% of each lease total value to pay Broker commissions, to help make sure she attracts the best possible tenants in her marketplace.
@Michaela G. will want to make sure the roof, and HVAC equipment are always maintained in top shape with annual inspections and service contracts. She'll maintain the landscaping with weekly service to keep it looking just right. She'll hire a Porter service to visit the property every day to sweep sidewalks, remove litter and debris as needed, and generally keep the property nice and clean. She'll hire a window washer to come clean at least 3 times a year and after every storm. She'll contract with a waste management company to provide a large dumpster at the rear of the property for tenant use which will be serviced 1 or 2 times per week depending on how much trash the tenants generate. All of these costs (and several not mentioned) will be pooled together into the Common Area Maintenance or CAM which will be charged back to each tenant on a prorata basis per rentable square foot (RSF) or some other means of calculating as may be negotiated in the lease. @Michaela G. will add a small management fee of 6%-8% to the CAM to compensate her for the effort of organizing all the services required to keep this small retail property a place where her (4) tenants business' can thrive for many years.
@Eric Thornton Your analysis will be exactly the same for an off market opportunity as one that is listed on market. If you're using one of the calculators just input hypothetical purchase amounts to see what they look like... play the "What If" game. "What if I could buy it for $680K?"... "What if they would only sell it for $1.2m?"
Take all the experience you've gained looking at and analyzing all the too expensive deals this past year to help you recognize your great deal when you finally uncover it. Dolf DeRoos says you need to look at (100) potential deals, make offers on (10), to buy (1). The ratios don't need to be exact but by the time you've looked at enough deals to make (10) real offers you will know for sure that the (1) you purchase will be a great deal for you.
Best of luck!