I don't know if it's even possible to build this item. I'd like opinions....
First, is the GP value of staves in teh book off? I read that somewhere but don't remember.
I would like to craft a staff that works as a spell storing item. I would like the staff to store 2x the caster level in spells.
So, if at 10th lvel wizard has the staff it would store 20 levels in spells, 11th lvl 22, etc....
I might also add another straight forward ability or two, but I can probably figure those out.
When I run a game these are the type of "heirloom" items I will often give someone at 1st level as a "special" item. Because it grows in power with the user they often keep them for their entire career. Nice as a GM =)
Staff prices are in the book. To make it spell storing, add 1.5 times the price of a ring of spell storing. Or if spell storing is all it does, then it's not really a staff, just a reslotted ring of spell storing and would cost the same as one.
To make it level up with the character... you'll have to take a wild guess as there's no existing items to compare to and nothing on the item creation chart. You could either price it at the max value (probably no one would buy it), at 80% of that, or possibly at the level 15 value. I don't really know what a fair price would be.
The staff prices in the CRB are accurate for formula and the APG staff prices are half what they should be. This has been fixed in Complete Equipment.
What you're talking about has no equivalent in the system. The best you could start with is a ring of spell storing. A minor ring of spell storing is 18,000 gold and can hold 3 levels worth of spells (so three mage armors, or 1 magic missle and 1 acid arrow). The spells stored in it take as long to cast via the ring as they would normally take and are cast at the lowest valid CL for that spell (much like a scroll). The normal ring holds 5 levels worth and sells for 50,000 gold.
To make a staff that can hold CL*2 would be... expensive. It would also not be a staff according to the rules of staff items. A staff has a list of spells it can cast and a number of charges used to cast the spells.
So, to build it always remember the 1st rule of the game - the GM is always right. You don't have to use the formulas to make an item; as the GM you can just say "here's an item and this is how it works, have fun!" Especially useful if it is an heirloom item with the caveat that it only works for them and they get horribly cursed if they ever sell it.
I was thiking there would be no way to build it as per Raw. Good advice though.
BTW, the rings of spell storing seem to follow the level squared * 2000 progression. The shape of wondrous items is usually not a big deal, as long as they still take up some slot. (The price doubles if they don't.)
Edit: That means that for 40 levels of spells, it'd cost 3.2 million.