I would never offer cash. Because now we're getting into legal contract territory.
Think about it. What's the purpose of paying the person to hunt their property? It's to give them something of value in return for your ability to hunt (and presumably keep what you find). In other words, to create an agreement of some kind, whether legal agreement or a gentleman's agreement.
What if you find something they want, even after you paid them the fee? You gonna give it back to them? If so, then why did you pay the fee?
You gonna keep it? Really? You're gonna go to war over something you found during the course of your hobby? You say yes. Then in that case, be prepared for the landowner to argue that you didn't offer them a fair price for permission to hunt, you too advantage of them, you knew what was there the whole time and you low-balled them, etc.
The only time I can see paying a fee to hunt as working out is if you agree to hand over all your finds and give them all back to the landowner if they want it. But in that case, I don't think most people would agree to go metal detecting on other people's property, especially after paying to do it.
You don't get paid by the trash collector so they can have the privilege of taking your trash away.
