Why are medicines in mexico are so cheap?

There is a lot of misleading information in the other answers, and a lot of that comes from fear mongering put in place by lobbyists. There are a lot of reasons why medications are less expensive, but let me use one example: insulin.

A lantus Insulin pen is $100 in the US. That same pen is $12 in Mexico. It's made by the same manufacturer, which is german. This has nothing to do with inferior quality. In fact, the same pen is also available in Canada for $12. Mexico does not require a prescription, as a result of less regulation, but that doesn't affect the price much.

The truth about the prices is more closely linked to more competition by allowing more companies to produce medication. The US also has regulations in place so that even if a foreign manufacturer were able to get approved, they could not undercut the prices of American brands.

The regulations on drugs are a lot more lax in Mexico. Mexico doesnt have an FDA so drugs can be used past the time we, in the US, would expire them, so they can be God knows how long, on the shelf and could be breaking down and less potent. They also do not restrict cancer causing agents so drugs there may contain substances that are cheaper to manufacture but banned in the US.as a carcinogen. They are not banned from adding lead based petroleum products and other heavy metals. The list goes on. Some of these regulations do make drugs more expensive in the US but it also makes them safer. It is your choice on whether or not you want to chance it. I know that it is tempting to save money this way but it could lead to more problems later on when you become toxic from heavy metals like lead and murcury which they dont regulate like we do in the states or you get cancer from carcinogens left in these drugs. Do your research and then decide for yourself what you are willing to put in your body. For myself, I prefer to pay the extra and know my drugs are regulated and safe as they can be.
It may take a little work on your part but there may be less expensive options for your US meds. You can try the manufacturer to see if they have a patient assistance program where you can get the drugs at lower or no cost. There are also organizations that offer deep discounts. Some of these are the Co-Pay Relief Program 1-866-512-3861 or www.copay.org; Suddenly Senior (you dont have to be a senior to use however) www.suddenlysenior.com/patientassistantprograms.html; Needy Meds www.needymeds.org; Partnership for Prescription Assistance 1-888-477-2669 or www.pparx.org are all organizations for helping people get the meds they need at lower cost options. Also check with your state. I know here in AZ we offer the Copper Card which gives drug discounts right at the pharmacy. Research your options-it probably wont take much more time than a trip to Mexico (and will be safer).