I would have flipped that around..."you called me? How much are you going to pay me to detect it?".

Charge him to detect it and he gets to keep all the finds! SO, how much does he think is actually in the ground? If he's willing to do that then he might really think something is there. Unless, he was just returning your call? That would be a little different. If your asking for permission. I've offered to split, or they pick. Most of the time I was doing it for fun. I wouldn't pay anyone.
I've never heard of that yet, I've heard of people doing it for free for historical places to put the finds on display. I've heard of splitting with the property owners and every other kind of deal. But never heard of paying someone per acre that calls you to detect. I'd mess with him a little, to try to figure him out.

Then just move on, there are plenty of places! Can't blame the guy for trying to be creative and make a buck though!
On the other end of it, if he was returning your call? He might have people asking him to detect that property. After a while I could see how someone might say."well.... How much are you going to give me?". My Dad lives on a historical property, we have hit it hard over the years. And every once in a while someone knocks on his door to ask to detect. Some he allows, some he doesn't, kind of depends on the day.

So, I could see him getting fed up on a bad day saying"How much are you going to pay me to look?".
