Yes, the taxis around the falls were quite expensive by Argentine and Brazilian standards, given the short length of the ride. The prices were also non-negotiable, I tried. If LAN had not screwed our schedule up we would have been able to do it much cheaper.

Our original plan for Iguazu was to spend one full day and fly out at 6pm on the second day, so have a full day on each side of the falls. Unfortunately, LAN cancelled the 6pm flight a couple weeks before we traveled so we ended up on a 2pm flight forcing us to try to fit everything in on one day and therefor we needed taxis.

Should our original schedule have held we would have been able to take public buses. From our Argentinian hotel we would have taken a bus from the main bus station towards Foz de Iguazu, after crossing the border we would have gotten off at the first roundabout, crossed the street and boarded a Brazilian bus to go to the Brazilian falls (and repeated the process in reverse to return). This cost would have been only a couple of US dollars.

To go from the Argentine side to the Argentine Falls you just take a bus from the main bus terminal. It's all very simple but does take some time.