If the person who buried it is now deceased if I am not mistaken the current property owner has more of a legal claim to the cache then the decendents of the person who buried it. Check your local law, but you would be surprised to find that in many states, decendents have less of a claim then the current land owner.
I would make a deal for no less then 40-50% unless the current owner is a close personal friend.
It will be easier to go with a 50-50 split, or to divide it into thirds.
I would be careful to get it in writing, you just never know.
Take care, and good luck!
Exanimo!
The Gringo
ps. Thanks for sharing, I was in an identical situation a few years back.
pps. You should offer her the 50-50 split, or 33 & 1/3% with a weeks pay & expenses up front, ect..... Presentation is key! Get your ducks in a row and go for it!