I personally would not take a "cut" other than the coins I wanted to keep. This kind lady gave you permission to detect her yard and has allowed you to search through her change hoard. In my opinion, you owe her...not the other way around.
This also mirrors my sentiment, BigWaveDave. Let me tell you a little story...
For months I had been driving past this old brick house with a for sale sign on the front lawn. The lawn needed cutting then, and after a couple more months, it got really high. I called the realtor and proposed a swap: My time mowing for permission to detect the lawn and woodlot. He deferred me to the son of the owner, who claimed that his job kept him from mowing and maintaining the place. He eagerly said, "Go for it!"
So I did.
When I rolled up, there was a car parked in the driveway. It was the owner, a nice old lady who had lived there for 47 years. Her son (who seemed less-than interested in his mom these days), neglected to tell her that I was coming by. She was happy to have someone who would mow and make the place look respectable again, and I was happy to get a chance to detect a virgin site that was built in 1817! Come to find out, it was an old stage coach stop, with tavern and farrier right there on the premises.
So I mowed and mowed and went back and fourth, getting the two-foot-high grass back into shape, all the while the hot sun beating down on my head. It was worth it, just to gain the woman's trust and recognition. I'll go back an mow it this week again, and do a little detecting after. So far I the oldest valuable thing I have found is a 1946 wheatie, but tons of other stuff. LOTS of horseshoe nails, obviously. I am no where even close to being done with this property and its mine all mine! <insert hand-wringing and muahahahaahaa evil laughter here>
I guess the moral of the story, BWD, is that we are all ambassadors for our hobby and anytime we can interface with the public in a generous way, we get repaid in spades. Maybe in the next jug of cents you go through, there will be a 1944 steel?
