As there is question as to the value of the cache, possibly a sliding split may be your answer. We use this for commission calculations in some of our auctions. In the Auction, if the sale is say between $1 - $100 - the seller pays 35%, $100 - $500 seller pays 25%, $500 - $1000 - Seller pays 20% etc.
So, to make sure that your expenses are covered, possibly you tell the LO that you want a 33/33/33 split if the cache ends up being under $1M. This gives you $50,000 each if it is $150,000 in bills. If it is between $1M and $5M you will move it to 40/30/30 split. Between $5M and $10M you will move it to a 45/27.5/27.5 split, and anything over $10M you will honer the 50/25/25 split.
This allows the LO to see that your intention in the split is more related to covering your expenses.
On the opposite side, if it is not just to cover expenses, but to reward your value to the endeavor, you reverse the the direction of the splits by cache amount, and it gives the owner his 50/50 if the cache is smaller, and rewards you the larger it gets.
I think that a face to face conversation followed by paperwork is the only way to go. Drawing a contract that ensures that you will not damage his property or hold him liable for any injuries etc, and even being bonded if requested by the LO.
Frankn - Is this related to a WWI draft dodger by any chance?