First item: there's a loophole in the costs. If you want a staff that has a single 8th level spell you can stick on two 9th level spells at 10 charges each and lower the cost from 48000 gp to 37910 gp. Needs careful wording to fix, but no deep game implications.
Second item: I think the costs for multiple spells in a staff is too high. Rather than being 75% and 50% it should be more like 50% and 25%, or maybe a flat 25%, or go even lower. Consider that, rather than adding an entirely new ability, you're extending the existing spell pool to have an extra ability. You're still limited to the (now rather small) capacity of that pool, and it should be the dominant factor in the price.
The base price should probably be increased to compensate. This pushes staffs further from wands, which is a good thing IMO. If staffs were intended to be "wands, only better" then why separate them in the first place? This could also make the level limit on wands unnecessary, as you have to pay for the mechanical benefit of staffs.
Third item: How the heck does the Staff of Power break down? Due to the aforementioned loophole the costs are ambiguous.
