The value of a piece of mounted taxidermy is always difficult to ascertain. The main points to consider are as follows.
Its current condition. Is it damaged at all?
How rare or unusual is the item? A Buzzard could be worth more than a fox, although the fox may have taken longer for a taxidermist to prepare!!
Which taxidermy firm originally prepared the work? Does it have a label detailing this? This is usually the most important factor in determining its value. If it was simply prepared by a hobbyist or game keeper it may not be of any great monetary value!