Its a one time fee for the life of your passport. It was $100 when I got mine. When you come down the escalator into customs, it is a bit confusing. There is the typical customs line that most folks go into. If you haven't paid the $100, take an immediate left off the escalator and get in that line, otherwise you will wait in the customs line then be sent back to pay then start the hole line waiting process again.
I think I have 8 trips down there now on mine. I have to get a new passport this spring though so I have to pay the fee again when I go down again in August.
Also a little tip, you will have all sorts of official looking folks offer to assist you with your baggage, unless you want to pay them or tip them, decline their services. They are quite pushy. In the past they were allowed into the baggage claim area and it was really confusing with who was security and who was just trying to make a buck. Now they wait outside the exit of bag claim and offer you help there. Just an FYI.
If you enter via overland, as we did on the trekoftheamericas' there still in NO charge....Peru to Chile
I understand that the resaprocity fee for entering chile is no longer in effect. So, you shouldn't have to pay anything for to enter. However, if you are doing some kind of standby flying, check the seat availability continually. With the new treaty/agreement between Chile and the States, more people are flying and I've heard that, Delta at least, uses most of their flights as embargo flights. This means seats can be empty, but cargo weight of the plane can knock you out of going. This happened to us when we went back to the states in Aug of this year (2014). So, be prepared to possibly not get on the plane for standby flights. Plus, more Chileans are coming to the states, so the planes are a bit fuller than they were a few years ago.