The price difference for in-state and out-of-state tuition at U.S. public universities is eye-popping. In fact, on average, out-of-state students pay a whopping $13,000 more in tuition each year, according to the "2012 Trends in College Pricing" report from the College Board. Every state has different requirements for out-of-state students to qualify for in-state tuition, and at an average savings of $13,000, it's no surprise that so many students are interested.

Establishing Residency

There are many different ways to prove residency for in-state tuition. Most universities require students to have lived in the state for at least one year, although that time period depends on the state. For example, Tennessee doesn't have a length requirement -- the only state without one -- and Alaska requires residency for at least two years. Some of the most common ways to prove residency include state voter registration, in-state vehicle registration, applying for a state driver's license, and paying state taxes. College Board hosts a comprehensive guide to each state's requirements.

State-to-State Discounts

Oftentimes, neighboring states have policies for allowing students to attend public universities and pay in-state tuition or deeply discounted tuition rates. One example is the reciprocity agreement among Minnesota, Wisconsin, and North and South Dakota, which offer reduced tuition rates for students in each state. These policies vary by state and institution, but more and more institutions in rural or remote locations or smaller colleges in competitive markets are using them as a way to bolster sagging enrollment. For instance, Southern Illinois University offers in-state tuition to incoming freshmen from Indiana, Missouri and Kentucky, and North Dakota colleges and universities offer in-state tuition to out-of-state and international students.

Tuition Exchange Programs

There are several regional tuition programs in the U.S., which are groups of states that allow approved students to pay in-state tuition in every affiliated state. For instance, the New England Regional Student Program allows potential students from six states to pay in-state tuition, and another program, the Academic Common Market, a network of 13 states, accommodates students in the southern U.S. Lastly, the Western Undergraduate Exchange and the Midwest Student Exchange Program have been established for students in western and midwestern states, respectively.

Immigrant Students

A number of states have agreements offering in-state tuition for undocumented immigrant students. California, Connecticut, Illinois, Kansas, Maryland, Nebraska, New Mexico, New York, Oklahoma, Texas, Utah, and Washington all offer in-state tuition to undocumented immigrant students, and a number of other states are currently discussing similar laws, including Colorado and Oregon. In California, students are also eligible for financial aid. Although the laws vary by state, most require that students have lived in the state for a specific number of years and graduated from an in-state secondary school. These students are also required to be working toward legalizing their residency status.