I got mine from 18 Bikes for £30, fully sealed extra long version. I had to drill out my cable stops to fit a full length outer which is something you need to consider.
Other than that I found them as easy to fit as any cable and after 2 or 3 months they are performing as good as new. “Normal” cables usually clog up after a couple of rides in the Peaks where I ride, but I do seem to suffer more than most with that problem. My gear fettling is now greatly reduced.
I have Nokon cables on my other bikes which are a bit smoother than the Gore, but then they do need lubing. The Goretex inner sheath in the Gore cables has little enough friction so as to not need lubing.