It won't be $20 when you arrive in early September. But the way things are going, it will reach that mark early next year. I had been predicting $20 by the end of the year, but the government always shores up the peso just before the main tourist season to lower the rate versus the dollar to make a vacation here look like a good value for tourists.
However, their attempt to lower the peso this past week by injecting $50 million US dollars into the economy (their traditional way of doing this) failed as the effects of this artificial lowering lasted only a few days and the peso climbed back up to it's current exchange rate. I heard it was already going over $18.50 in some places,...and climbing more.