Depreciation is a fact of life just like death and taxes. Trying to quantify it is impossible. The IRS gives guidelines on depreciable capital equipment and real estate but that has absolutely no reference to market value which is what your really interested in. Fair market value is defined "What a ready, willing buyer will pay a ready, willing seller.".
The pricing people check sales prices for everything on the road and display the results on their sites. These didn't used to open to the public so hoorah for the internet. If you want to know what a coach is worth go to NADA or Edmonds.