having just returned from Bs.As I have to comment on two things. One is I can not imagine giving a taxi driver a $100 peso note. (would you give a taxi a !00 bill for a 10$usd ride in NYC? ) Most rides were 5-7 pesos. No cab ride we took over two weeks cost more than 13pesos and that was from Recoleta to Villa Urquiza (about 50km) It is foolish to not have small amounts to give to a taxi driver. The other is that the regular Radio taxis to and from eze to centro should be about 45pesos (15usd) including toll. Ask before getting in at the airport.
75 pesos is way out of line.
As far as crime we were out every night until 3-4am took radio taxis and spent lots of time in areas with very few tourists (thats what tango in local clubs every night does to you)
Use common sense as you would in any city and you will have no problem.
Buenos Aires is a great city have a good time.