If you posted this "offer" on a non-treasure hunting forum, then I could see many people agreeing with you.
Posting this here really means that you are not that special as there are many who could do your job, maybe even better!
A lot of us think that we could find such a cache on 9 acres of land.
As for your research and time, I bet I have spent MORE time researching and searching for historic sites for NO financial gain!
How do you come up with the $15,000 figure of what you claim you have sunk into this project?

Is that gas money? Your time for research?
There are tons of people on this forum that will spend an entire day digging pull tabs in hopes of finding a $50 gold ring.
What I'm getting at is that there are thousands on here that would work for practically nothing. That's why lost rings are found and returned with no expectation of a monetary reward.
What is stopping the land owner from looking up a local metal detecting club and dangling a carrot? I could see 50 club members showing up just for the thrill of it.
I'm saying that there are thousands that would easily underbid you! Especially with this current economy.
Now you are talking about several million dollars worth. Word gets out to the landowners and they are easily in a position to seek a better offer elsewhere.
Heck, I would spend an entire summer camped out, just to work for a fraction of what you expect!

It's your prerogative to ask what you feel you deserve. I really do hope the best for you!

I just wanted to remind you that you're not the only one with research skills, experience, time, and a metal detector!

Take the deal before it's too late.