On myhorsevalue.com they will ask the location, age and sex of your horse along with a rating that you would give for the horse. Once you hit “enter” then a price is calculated based on the average price of horses recently sold. However, there is no indication of how often their database is updated and where there data is derived from. (a mystery)
Even without verification of the data, it is a very useful tool.
Of course we all know, that the ultimate value of your horse is determined by the seller, and the buyer that agrees to said price.