After the Battle
OOC: Power doesn't "store" between "owners". The moment you 'make it yours' (which by RAW is "claim it, keep it, and pay to recharge it") it loses the charge the previous Owner had in it, as Power Items are personal. Which means once it's charged, you can't hand it off for someone else to use the Power from, it's only yours. That also means if someone steals it, and pays to have it recharged for themself, your charge is gone.
And yes, it's impossible to know how much until you get it appraised (but we can make educated guesses as Players*).
It's possible (outside chance if he's of the Knight Template)) that Aegis has Connoisseur (Weapons) or Armory (Melee Weapons), both of which wold allow him to make an appraisal roll.
* It's a Staff, it's Ornate, so that's at least 20$ up to 100$, so 1-2 PF. Further it might be Fine (+20$), Very Fine (+190$), Sylvan (+150$), or Balanced (+40). It could be Silvered as part of the Ornate, and of course Ornate just means "make it more expensive" so it could have gemstone set itno it, etc... so it's anywhere from 1-5FP, but it could go higher. And yeah, staffs suck as Power Items unless you swap out the wood for Dragonbone... because bone is expensive.
That's where the Alchemists really make their money, peddlin massive bone tonic to Wizards...