Did this last week. We were quoted 300 reals for a taxi round trip. Ended up hiring a hertz car and driving over for 24 hours for 100 reals - we shd also have gotten a carte verde (3 day insurance policy) in Argentina for 45 reals or so but couldnt find the sales point so, as it was late, just took a chance (not advised!). Our decision was last minute so it shd be easy to identify cv locations thru google and email with time. Driving and border procedures were straightforward in both countries. NB many of the taxis we saw wd be too small for 4 people and baggage so you may need to prebook.
Felt that argentine side was much more spectacular and jungly so wd maximise time there. However, the bird park next to brazil np entrance is very good - you can walk through large cages with macaws and toucans (who gently peck your shoelaces or backpacks!) We swallowed hard and stayed at Sheraton iguazu within the arg national park which was worth it as it gave a vital few minutes start on the crowds in the morning - we had the lower level path to ourselves. (April shd be quiter than jan tho). It also has a shuttle to brazil airport but dont know cost. Arg side much cheaper than brazil. We thought iguazu as a whole was fantastic and wished we had scheduled 3 rather than 2 days.
Note that there 2 airports, one arg and one brazil so you cd fly into one and out the other.