You might have to rethink your budget then...I m serious, riding is really expensive here, and if you want a private lesson as well as a horse for the lesson (so you dont have your own), that is probably the standard price you will pay anywhere, more even in the Zurich area. Group lessons will be cheaper, but still expect to pay between 50-70 Fr for those as well (after all you do get the horse and all the material to use).
I ll still wish you luck in finding an ideal stable for you! it can be hard.
I believe CHF100 is not expensive. I was doing it in France. Private teacher costed EUR75 per person (it was 2 of us), so in CHF it should be around 100-120.