Compare the prices of the following two staves:
- A CL 17 staff with a single 8th level spell (that costs 1 charge) would cost 108,800 gp (8*17*800)
- A CL 17 staff with an 8th level spell (that costs 1 charge) and a 9th level spell (that costs 5 charges) would cost 106,080 gp (9*17/5*800 + 8*17*600)
This is pretty trivial, but I'd suggest that the most expensive spell effect should cost "full price" rather than the highest level spell effect.
Except that with staves only having 10 charges (rather than 50) that level 9 spell isn't going to have nearly the effect that the level 8 will. (2 castings of the first compared to 10 of the second)
Except that with staves only having 10 charges (rather than 50) that level 9 spell isn't going to have nearly the effect that the level 8 will. (2 castings of the first compared to 10 of the second)
You're not getting the "trick" -- you would never actually use the level 9 spell at all; it's just there to make your staff less expensive. The discounting effect would be even more pronounced if the DM allowed an effect that took 10 charges to activate.
Although it just occurred to me that the staff with the level 9 spell would take a higher level spell to recharge. Maybe it's not as bad as I thought.