You can see my diatribe on tax values here:
Buying below tax value is no indication of a deal....nor is buying above tax value means you are getting a raw deal. Either way the retail purchase price could be too high or too low. You need a realtor to help you look at the market. Tax values are only an indicator of what tax you might pay over the next year. I would almost never use tax values as an indicator as to what purchase or sales price should or should not be.