Well, the rest of the story is:
My wife called me and said "hey how much is this worth?"
So I grinned, and said "Well, $11 or so. Are you going to her house to ask if she has more?"
(the lady was someone my wife knows from the neighborhood)
She said "Well, yeah! But what should I say? How much should I offer?"
(good girl!)
I said "Tell her 'My husband collects coins and I noticed you paid with an old coin. He's going to be very excited to get that! I would love to purchase any more of these you have. I will even pay you quite a bit more than their face value if you would like to sell some.' So you can pay her 20x or so on any halves, quarters, or dimes which is better than she would get at any local coin or pawn shops."
She went back and the lady didn't know. "It was just in my coin purse, and I thought the girls would like the half dollar."
"Do you have any more, ma'am?"
"No, honey, I don't think so."
"Well can I pay you more for the one you gave them?"
"Oh honey, no, that's quite alright."