You'll find lots of arbolitos (the people shouting cambio, cambio, in case you didn't know) in calle Florida on Saturday, but you may want to ask the price from five or six of them or more. The price can vary much more on the weekend than during weekdays. It's even worse on Sunday.Thanks all - are cuevas open on Saturdays?
We arrive BA late on Friday night, stay 2 nights, and then Sunday very early fly up to Iguazu, and get back to BA on Tuesday night; I'm planning on paying hotels in cash for those first 4 nights, so I need to get a stack of cash on Saturday!
Also, try to size them up as you're talking to them. If they seem too shady, walk away and try another. And ask them in the street if they have large bills (mil [1000], hopefully, or at least quinientos[500]), because sometimes you have to walk a little way, and then into the building where the cueva is. I've always found the arbolitos in the blocks close to Avenida de Mayo to be a little more shady than further north on Florida. The last few blocks are a bit sketchy as well.