Interesting issue. If i had that issue, I would offer 50/50 split and not talk about the possible value. Despite your intensive research and all the indications you may have, it may not be there. When you start to talk about having some real money on the table, the bigger the amount, the more the owner will feel entitled. No need to bicker on a "if", but do have a contract in place stating a "possible" nominal value ($100) and up.
I would take a 50/50 split all day long. Here is why. For about 15 years, I was in the telecom business. I dealt with a large fortune 500 company that would send me tractor trailer loads of telco and computer equipment. I would inspect it, test it, refurbish it, re-package it, and resell it. The split was 65-35 in their favor. They sent the stuff, I took care of the rest.
If you think about it, this is essentially the same. They own the land and you are providing a "value-added" service.
I would be hesitant to start the negotiation at 33% for him. That number is low enough that he may feel that you are trying to pull wool over his eyes. If that happens, your credibility just went out of the window and getting him back on the negotiating table will be very tough.
If I were in the owner's shoes and you would approach me with that deal I would say thanks but no thanks and there would be no further negotiation.
If I didn't know anything about MD, I would take an ad in the paper or in a MD forum. My ad would simply state that I believe that there is a treasure on my property and that it will be open to all MD in the region on such and such a date. Free coffe and donuts. If you find it, you get 10% of it. I bet you I would have 100's of MD on my property on that date.