Taxis are the best way to get around Monterrey. They are pretty cheap and are usually never more than 4 or 5 dollars to get around the city. If you dont speak Spanish try and write down the address where you are going because the driver probably wont speak English. Most places (Hotels & Restaurants) will call a taxi for you if you ask.
Traffic moves pretty fast around here, as noted by my last taxi driver, who prayed under every overpass. We talked about the difference between driving and in the US, and noted that it is much different. I'm used to it, and told him so, then he asked what I thought about Central Americans. I, of course, said I liked them a lot and he took 10 pesos off our bill and hugged me. What an evening. Other drivers have been slower, so they all aren't as fast. But a ten minute ride can cost about $60 pesos. Some taxis are green, but there are others that are not marked.
Getting around Monterrey is relatively easy. All you need to do is walk to the street and hail a taxi. These guys get you around Monterrey QUICK, but be careful because out of the fifteen or so taxis we rode not one had seat belts in the back seat. Make sure that the driver sets the meter whenever you get in or they are liable to charge you double what the normal fare would be. Fare for us ran us anywhere from 10 pesos to 40 pesos, pretty cheap in our eyes.