Buzz, sometimes it is not that easy to sort out hormonal issues; such as in the case of nymphomania, where a mare will show signs of estrus all the time. If the symptoms aren't too severe, things like regumate can help, or even keeping the mare bred can help reduce symptoms. But sometimes it is severe enough to where no other option can help, and only spaying can; (in some instances even a complete spay is not a guarentee). There are other issues that can lead to spaying of a mare too, from constant ovarian cysts, to cancer. It's not unheard of, and certainly not 'outrageous' if the horse that valuable to the owner.
OP, your mare is stunning! I hope you can figure out her 'mare issues' without having to resort to surgery!!! I'm sure she's miserable too, when she feels that way!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."