I just bought Sketch (student license) and really found it worthwhile, even if I were to only use it for that one application. I initially purchased 'design + code' on bitfountain, which I found helpful, but YMMV.
It does a fair bit more than just designing icons.
I just wondered if it's not cheaper letting a designer design the few icons an app needs
Probably not, especially not if they are custom made. However, if it's only icons you need, I would check out FlatIcon. Most of their stuff is free for use. There are a lot of free icons out there, it just may well be that none of them fit your usage.
wow this flaticon is great
By the way besides icons what is sketch being used for
I have personally only used it to set up a custom cell and to create and app icon. Both of which we easy to learn how to do, coming from absolutely no interface design experience.