1)   The *VALUE* of the property is irrelevant.

2)   The cost - what your client PAID for the property is needed (unless the FMV at the time it became a rental was less than the cost).

3)   Unless there are recent sales, Zillow is virtually worthless.   Per Zillow, my house only has one bathroom....

4)  If it's a recent (last few years..) purchase, the title company should be able to provide a closing statement.

5)  What state?  In some states (CA) the property taxes are based on the historical purchase price.