The difference between the price paid by a non-Texan US citizen and an illegal alien is certainly free.
I am not sure why we are having trouble with this concept. The "illegal" part has NOTHING to do with "residency". If you have been a resident of Texas (and thus paying Texas Sales Tax and Texas Property Tax) for the past three (3) years, you receive in-state tuition ... regardless of what the people in D.C. reflect in your immigration file. Period.
Any "non-Texans" who choose to move here for 3 years can also qualify for in-state tuition.