So say a 4000m launchpad is feasible, that's 2% of the needed altitude. While gravity will be roughly the same (yeah it's higher altitute, but there's the mass from the mountain). There is much less atmosphere* (~50% less pressure & 1/3 less of it to get through).

Given that payload to fuel ratio is roughly 1:10, this does seem to add up to a significant saving.

*Air pressure numbers http://www.mide.com/pages/air-pressure-at-altitude-calculato...