I think with a 15 year old dog there is a lot to balance.
First, as mentioned, the risks of surgery, and if that is a risk that you think is worth taking. Any surgery is risky in a senior, but many seniors are pretty resilient.
The next is the money thing. I look at it this way. If, heaven forbid, something were to happen 3 or 6 months down the road, would you still be glad that you had the surgery done? Would having this surgery mean that if something comes up down the road, you would not have the finances available to address that?
For seniors, things can change pretty quickly. Some surprise us and live many many years longer, and others don't. I think that so much of these decisions are very personal, and you have to do what you think in your heart is right.
Best of luck,, in whatever you decide!