What about the Canal Grande in Venice? Normally you'd buy some kind of travle card, but a single journey on the Canal Grande will set you back 5 Euros. And bear in mind this could be just a few metres to the other bank. Having said that, the three-day pass for (I think) 22 Euros is one of the best deals going.
Panama could be correct. I went through on a cruise in 2003 and the fee was $150,000. That works out about $3750/mile. As the fees are worked out on tonnage of the vessel, the container ships etc probably pay more than this.