Like Eastbay1K, I've gotten my money's worth out of my reciprocity fee receipt. The one in my old passport cost me $20 in 1995, and counting up the stamps in that passport, I see I entered 25 times with it.

En estricto rigor, the receipt is good for the life of your passport, but if you're lucky, it can be good for longer than that. The first time I entered Chile after I had renewed my passport, I expected to have to pay the reciprocity fee (which had since risen to $100), since the $20 receipt now had holes punched through it like the rest of the old passport. But after some consultation between the fee booth person and the passport control people, I was given a new receipt without my having to pay.

A few months later I got my permanent residency card, and sometime soon thereafter, either on leaving or reentering Chile, someone in passport control removed the $100 receipt from my passport.

In the new passport, by the way, 14 entries so far, and counting.