Where you live, where you study and how expensive it will all be will depend on what you want to get out of the island. Some people like to live near the beach and surf every other day. Some people like to live in the mountains and plant fruit trees and farm chickens. Some people like to live in Old San Juan and pretend that they're bohemian artists in a Caribbean version of a Spanish city. Some people like to stay close to the cities to pursue a career. How do you see yourself living on the island?
What do you want to study? This might narrow down your choices. For example, if you want to study engineering you might end up in Mayagüez in the west. If you want to study law or arts you might end up near Rio Piedras in the metropolitan area. Public universities are cheaper than private ones and traditionally offered an education of a higher quality. But this is starting to change. Some universities are halfway decent but some of them are just glorified high schools. But your degree will be valid in the USA and a Pell grant will likely cover your full tuition.
Do you guys have useful work experience, a trade, or specific skills?
Puerto Rico is not Latin American cheap. The cost of living in the island is closer to the cost of living in a poor neighborhood in the States than it is to the cost of living in a good neighborhood in Costa Rica. Keep in mind that the wages are proportional to this.
Public transportation sucks, specially outside of the metropolitan area. Depending on where you live you might want to invest in a car. You can buy a cheap used Japanese car just about everywhere and the cost of liability insurance is a joke.
The better your Spanish is the better your chances will be. You could live in the island with zero Spanish with few problems, but getting a job or close friends will be more difficult. Force yourself to talk in Spanish with your fiancé and rent some Spanish language movies. Your Mexican fiancé will be an invaluable resource. (Mexican Spanish is similar to Puerto Rican Spanish, but not exactly the same. Did he ever tell you that soy sauce is Salsa too?)
If you decide to do an exploratory trip to the island I recommend that you rent an apartment for a week. It will be A LOT cheaper than staying in a hotel and many tourist oriented apartments are owned by expats who might give you some extra insight.
There are some communities where the concentration of expats is high. You might want to look into this. It helps many to cope with feeling homesick or isolated.
Night life will vary. Puerto Rico is not as homogeneous as it appears to be. You will find local Salsa, Dominican bachata music, New York style artsy hipster dives, ocean front bars that blast nothing but Journey, many wannabe gangsta cacos, etc.
My biggest piece of advice is to search for Expat.coms and to contact the authors personally. I could link you to a few if you want. And feel free to ask me any more specific questions or look me up in facebook, etc. (I am very easy to find.) I like gringos because unlike most of us, most of them don't take the island for granted.