I've appraised several commercial properties where the owner had a long term property tax abatement in place. Generally I would provide the client two values, one with the assumption that the abatement could be transferred to a new owner, the other assuming that it couldn't be. The difference was the value of the abatement. I determined the value by looking at how much they were saving annually on taxes, projecting that into the future with some annual growth (2-3% typically), then discounting it all back to present value. On the last appraisal I did I assumed a 2.5% annual growth in property taxes and used a 7.5% discount rate to determine the present value of the future savings. The assumption is that a buyer is basically prepaying for those future savings and why would you pay say $20,000 up front to save $25,000 in future property taxes when you could simply invest that $20,000 and have $30,000 - $40,000 over the same time frame with a reasonable amount of risk.