I had to put down a rescue horse afflicted with HERDA a couple of months ago. He was boarded at a facility with young children, so in order to spare the family having to have a dead horse on the property or deal with the removal of the body, I hauled the horse to our local vet hospital, where they put him down and took care of the rendering. The cost was $350 - but the cost does vary based on the weight of the horse (this was a very thin pony stallion).
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Then to add insult to injury, they mailed me back her halter 3 mos. later. That started the waterworks all over again. </div></BLOCKQUOTE>
I've had horses shot and PTS by injection. Shooting may seem brutal, but as long as the person doing it knows exactly what they're doing, I much prefer it to the injection. When I had to have Pip PTS on course on XC (he smashed his cannon) I chose the gun.
Jingles for the filly though. I'd phone local vet clinics etc and see what they say. Good luck.