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Not much at all. Low earth orbit is ~200km so the mountain is only helping by 1.5% of the needed altitude.

Altitude is not even the most difficult part of getting to orbit. Getting to the necessary horizontal speed to stay in space takes a hell of a lot more fuel than going straight up.

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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...

you're missing the point. altitude and drag losses are nothing compared to horizontal velocity. you need to be going 7.8km/s. Per second! That takes a lot more energy than gaining any kind of altitude or overcoming atmospheric drag. It's not significant savings.

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