I think this solution also misses the fact that certain kinds of people (like bored kids) will walk through fire for a free quarter. It's not even the money, it's the novelty. So if you have a population of people who consider the charge 'the cost of shopping' and don't care enough about 25 cents to return the cart, you still have a whole other population of people who will hunt them down and return them for those people.

As a kid, I almost missed a flight while hunting luggage carts at the airport.