Thanks, Bunni. I found another Chilean site that described the fee more fully. It's $100 for those who have a US passport, a different amount for Australians, and $35 for Canadians. The fee is only for those who fly in, and is collected in your country's currency, cash. And they want crisp new bills, according to what many have posted on Cruise Critic, nothing worn or dogearred. You pay it before you go through customs and before you collect your luggage. However, I'm told that it's for those who arrive by air only, not for those who arrive by ship.
So, if you fly to Santiago before a cruise, expect to pay. On the other hand, those of us who will disembark a ship in Valpo and fly out, or who are back-to-backers in transit, there is no "reciprocity" fee.
However, those of us flying out aren't off the hook completely. People who recently returned from a tour of Chile said that there is a $26 departure fee, payable in your own country's currency (for me, crisp new US dollars) at the airport checkin counter.