In Colorado, a "water share" is a share of water rights in an irrigation ditch company -- most ditch companies date from the 1800's, when the original settlers diverted stream water into irrigation ditches for agricultural purposes, with an incorporated ditch company to maintain the ditch, headgates, and water resources.
The way to find out what a share of ditch rights is worth is to call the manager of the ditch company -- they usually know the market value of the shares. The last time I had to do it, I was able to identify the ditch company through the county records. Water rights are a big deal in the West; the old saying, "whiskey is for drinking, water is for fighting" is true. Many ditch companies have had their water rights condemned and appropriated by municipalities, which have the highest standing in Colorado. The priority here is municipal; then commercial; then agricultural. Beyond that, appropriation seniority is "first in time, first in right." In dry years, junior right holders get nothing, while senior water rights get their full share.
Another thing to bear in mind -- every ditch company has different allocations per share. In one ditch company, a share might be an acre foot of water per year; in another company, it might be ten acre feet.
