I've attached my reply to someone else enquiring about hock athritis as there are so many posts from people about bone spavin, and I can only repeat myself so many times without losing the will to live
I wouldn't bother with the injections, in my experience they don't last very long and can cause laminitis. They can be quite expensive, especially if you have to repeat them over many years in a younger horse.
Hock fusion using ethanol is much more effective and is a once only cost. here is the link to my reply, scroll down and you will see it.
Ok I have a bill here from a couple of weeks ago so I can tell you the 4ml depo medrone cost £76.96 but I didn't have HA. The act of injecting into the joint was another £72.32. However with all the other bits and bobs they bill you for like visit, IV sedation, injection, gloves etc it was £240. This was for both hocks. Hope that helps.