Or if a competitor made such a hypothetical $25 replacement for the TI-84/86, schools could just standardize on the new model. The argument for not switching to Casio, etc. right now is that younger siblings typically get their older siblings hand-me-downs, but if the replacement model was only $25, that argument would loose a lot of weight.
Although with the Ti's current tenure, they're now getting into the range where there's likely students using their grandfather's hand-me-down calculator in class. I know students were using their parent's hand-me-down Ti calculators when I was in school.... and I'm old enough now to have kids of my own in school