Caster level is a bit weird for crafting staffs, but I think it refers to the lowest caster level necessary to cast the spells. There are a few conditions though:
1) The caster level must be the same for all spells
2) The minimum caster level is 8
Using the formula stated in the DMG and these guidelines (also stated in the DMG), I've successfully reproduced the Staff of Charming cost, but using the same guidelines, I'm a bit off for a Staff of Abjuration. Additionally, the highest level spell in a Staff of Abjuration is Repulsion (Clr 7, Protection 7, Sor/Wiz 6). Since it's a 6th level sor/wiz spell, I initially thought that the caster level would be 11, but the listed caster level is 13.
I'll post again if I can figure out the exact pricing, but the two conditions I listed above are correct.