bucnet.com // Personally the values seem high to me, but that may work in your favor for the current situation. You can also have a surveyor given you a valuation survey. This works for insurance so I''m sure it would also work for the tax man. Good Luck.
It has been my experience that prices listed in Soundings or BUCnet are on the high side and the laws of supply, demand, and human reaction to an uncertain economy apply. Take at least 10% off the listed price.