Neko lived a little over 4 years past her SRT (stereotactic radiation therapy). And it cost WAY less money than Cyberknife cost at the time and then was only available at Yonkers. You know that saying ECID or every cat is different. It's even more so with acrocats. The best outcomes for acrocats is a surgery called hypophysectomy, or removal of the pituitary gland. But you only want that done by a very experienced surgeons as the risk is also higher. The best most experienced are at the Royal Vet Clinic in London. There are only two that I know of that do it in the US, one at AMC in New York, the other at Washington State University Vet School. The prices I've seen for WSU are very high ($17-20K). For the price you were quoted, you might get a nice holiday in London at get surgery done there.
If you don't have that sort of coin hanging around, there are other options. SRT is still a valid option for less money. More details and outcomes expected in
this paper. Note, I was told an average of over two years when Neko had her SRT, but that also included some cats who died of other conditions, such as cancer, in that time period.
This paper by the folks at CSU where Neko had her SRT says 2.9 years. In Neko's time, SRT or Cyberknife were the only treatments available.
Any chance you could get the vet to give you a copy or link to the study? I like to collect that sort of information for the next person asking questions such as yours. Besides life expectancy, are questions like insulin dose changes, quality of life, and possible side effects from the treatment.
Another option a lot of people here are pursing is a drug called cabergoline. It's a drug given either daily or every other day at home. It's the cheapest of all the treatment options and requires no travel. Lots of posts about cabergoline in our Acro/IAA/Cushings forum. If you want to read a good paper on it, and pass it onto your vet, see the paper attached to
this thread. You can read some of the posts in the Acro forum from people whose cats were on it. It's the one a lot of people here now use due to cost while still helping the symptoms and giving better quality of life. I think there've been 4-5 cats go OTJ on it too.
Do you mean, what is the life expectancy for cats that receive no treatment? There are other options as I listed above. But there is no one answer to life expectancy with or without treatment. Besides the possibility of other diseases, there is just a lot of variability. The longest I've seen is a couple cats at 4-5 years, but also more at less than 1.
You may with to post in the acro/iaa/cushings forum, as a lot more people with experience in those conditions monitor it for traffic.