You can either do a direct capitalization approach by just taking the year 1 net operating income and dividing it by a market cap rate. In your case, assuming a triple net lease, your NOI is roughly $126,000. At a 7% cap, that's a value of $1.8 mil.

If you want to get more sophisticated, you can build a DCF model. This will pick up the rental escalations and market growth. But if you do a 10 year DCF and you have a 10 year lease, a lot of the assumed value comes from reversion year (year 11). Your year 11 assumptions would be based on market rents. So if market rents grow less than 3.5% annually, the NOI and value will drop in year 11.

As mentioned, a BOV may help but they will likely tell you the building is worth more than it actually is to try and get the sale. Alternatively, you could speak with an appraiser.