There is no such thing as stable prices in Argentina. Usually there is more than one price even for currency exchange, and inflation is rampant and has been all my life. Therefore, the question is hard to respond. I advise to check the local forums to learn about current prices, and then check the current currency exchange rate. If you are getting your money our of an ATM, your exchange rates will follow the official price. If you are carrying cash and can exchange it in the Black Market (super easy), you can get the Blue price, sometimes more than twice better.
I am a native and in my last visit used a local phone number. I paid between 15 and 30 dollars (at Blue rate), but every time I paid 30 the hour became an extended session.
If you check this thread in gayguides, you will find much more details about local prices.
Some of the boys, once they learned I was living in the USA, asked for 50 bucks.