[jmcomets]

With my very basic understanding of orbital mechanics, higher orbits require more dV simply because it's further from Earth.

The more interesting bit is that subtle orbital adjustments require much less dV in higher orbits than in lower ones.

I like to think of it like pushing a barrel uphill: it takes more effort if the slope is steep, but then once it's up there, it's a lot easier to get the barrel moving downhill.

Anyways, the "de-orbit" cost follows the same rule of higher being more costly in dV, the big difference with an ascension being that the atmosphere slowing you down is what you want (aerobraking is the word, I think?). So the difference in dV between a high orbit and a low orbit descent isn't proportionate to that of the ascent, if that makes any sense.