To supplement SJ's response; Since about 2010, converting any larger amounts of foreign cash, especially US dollars, to Mexican Pesos has been largely illegal in Mexico.
Although mostly ineffective, the policy was established, probably at the behest of the US Government, to help erradicate conversion of US dollars imported by the huge and powerful (and very dangerous) drug cartels in Mexico. The policy originally only applied to amounts more than $10,000 (US) but has filtered down to the point Mexican businesses and citizens are becoming afraid of taking lesser amounts into their banks and being charged a fee or worse, being investigated for money laundering.
Right now, the exchange rate is about 13 Mexican Pesos to 1 US dollar. But, your going to find that the street value of your US dollar to be nearer 10 Pesos to $1 (US).
Look, your $100 US is no big deal and you're not going to notice much loss when exchanged in Mexico. But it does point up the enormous changes that have taken place in Mexico over the least 5 to 10 years. I've traveled Mexico from border to border and Sea to Ocean both as a commercial pilot and as a tourista and the best advice I can offer is to use caution at every point.
Most of the larger tourist oriented cities and towns are probably safer than major sities such as Mexio City, but nonetheless, newspaper headlines commonly and routinely report large quantities of headless or otherwise mutilated bodies thrown in the town square by the murderous cartels. No one wants to be in the wrong place at the wrong time.
Sad, because interior or small village Mexico used to be one of the most welcoming and genuinely warm countries to visit... I made lots of good and enduring friends... all of whom advise against coming again...
Travel safely, but watch your back and always be aware of your surroundings...