Right now at interrail.eu there is a 50% flash sale on for one month continous travel at 335 euros for the whole eurail coverage area. Believe this is just interrail passes, for European residents only. For Switzerland it doesn't cover past interlaken to get up into the Berner Oberland, but does give half off of Lake Lucern boat trips and it claims totally free for Thun and Brienzersee boats. So you could use pass to get to Interlaken, then have a small ticket to get into mountains, which would be full price of course because half fare card doesn't make sense most likely. Not refundable, usable up to the next 11 months. Sale lasts for the next 29 hours. Time for some hurried math exercises. The non refundable part scares me, if it was usable up to the next 2 years or so, I'd get it for sure.
Just random checked a few mountain rail tickets, looks like 25% of Jungfraujoch, and 50% Rigi.