It seems like Deeds of Trust's for real estate properties (which are public records) include the loan amount on them (assuming a loan was obtained for the purchase of the property). Obviously this loan amount would exclude any down payment but it seems to me that if a neighboring property was purchased with a loan for twice the amount of the tax appraised value of the property that it's not unreasonable to ask that much from a prospective buyer as well?
edit: the actual sale price is not available in the state of TX. Quoting http://www.traviscad.org/faq_Misconceptions.html ,
The Travis Central Appraisal District does not have access to all sales information due to Texas being a non-sales disclosure State. This means that real estate sales transactions are not given to the Appraisal District. Each appraisal district must research all available data in the market place by contacting realtors, brokers, property sellers, and buyers to obtain sales information. Through this process the district receives some of the sales, but not all. Any and all sales evidence you can provide to the district will ensure proper valuation of your property.