I have had the same tutor for over a year now and I pay $20 USD per hour, paid into a USD account in Florida. I don't know what he charges in pesos but I think the going rate is about 100 pesos/hour.
At the language schools, private lessons were on average $25-30 USD/hour.
I found the classes at the language schools to be expensive and to be quite demanding in terms of time. They are about $225/week (M-F, four hours a day for a typical program). However, I also went to VOS (in Recoleta) because they were one of the only schools with a less intensive schedule. I went three afternoons a week (3 hours per afternoon) and paid about $100 USD/week.