Assessed value is totally different from what the property is really worth. Every 2 years they reassess the value of property in St Louis County. The ending value is not always based on reality. I spent a couple months this year helping my clients get their property values corrected because the county in it's infinite wisdom 4 years ago did drive bys - your property is in this neighborhood it must be worth this much regardless of condition or other factors. There was a HUGE outcry, so this cycle, they let the computer decide - I had clients with slab homes that were assessed at the same value as homes with basements, I had homes without pools assessed the same as homes with pools, etc. One thing to keep in mind with assessed value, is that if the county hasn't had a reason to re-evaluate the property, the starting point may be low and then the value each reassessment cycle is typically a % increase to that starting point. So if you have someone who starts out low, or who successfully fights and keeps a lower level one year, then the next cycle they are starting out lower than their neighbor is who has an identical building, so they stay lower. I can show you many homes that sit side by side that have widely divergent assessments.
Other factors that you need to consider is that a commercial building may have "other" property as part of the sale (for example a restaurant may have kitchen equipment and table and chairs that area included in the sale. The assessed tax value is for the land and the building, and does not take that into consideration.
If you are buying an established business, you are also buying their reputation, the expectation that the clientele will continue to come to the business and sometimes some trained staff or experience from the seller, all of which are not included in the tax rate.
Now, buying a commercial property is more complicated than buying a residential home. Unless you have a lot of expertise in the real estate field, you should't be doing it without a trained expert to guide you. Your realtor will have a fiduciary responsibility to protect your interests, help you determine a fair value for the property and help you to negotiate a good contract with good terms as well as price (sometimes the terms are as important as the price). The Realtor will also then help guide you to a successful closing, and ensure that all your inspections, title search, survey, etc are done in a timely manner and that any problems are identified and dealt with prior to close. Without a Realtor helping you, you are leaving yourself liable for not having a clear title, for bills that may be due after close and any number of other issues.