I just did this this past year.
My older competition horse had one that had been small for a number of years near his eye and suddenly started growing (was about golf ball size by the time we were treating it)
We were originally supposed to do 3 rounds of Regression injections at $200CDN per treatment. Did the first set and then found it was discontinued in Canada although may still be available in the US. The first treatment did take it down about 50% which we were very impressed with.
I did a bunch of research on different methods after that and decided on Cisplatin injections. Which was a bit cheaper at $300CDN for the last 2 treatments. However, you have to be careful as there is a water based one and an oil based one and you need the oil based one so that the treatment stays in the area longer. Also be careful with these injections as as long as it is on the area it can be dangerous to the handlers of the horse and can cause a number of issues the biggest one being kidney damage in people. I simply provided the staff with a box of rubber gloves and instructions not to touch it, and they used rubber gloves when taking his halter on and off ect…
If you PM me your email I can send you before and after photos if you like.