Why do the Texas Instruments Graphing Calculators cost nearly double when compared to the Casio Graphing Calcu?

I was at the store today looking to buy a TI Graphing calculator noticing that most of them were priced at right around the $100 range, I than noticed that the Casio Graphing calculator was priced at right around the $50 mark. I compared the two of them and it seemed as if they both had all the same features, etc. So why does the TI graphing calculator cost so much more? Are they that better than the Casio? Or does Texes Instruments just have their up there BEHIND?