I believe the connectivity kit is generally priced at $15 (at least it is at OD) minus the TI $15 rebate you can find by going
here.
The TI-83+ has advanced statistics functions that AP stat classes in high school require, but it is definitely a bit weak for higher level college math/engineering/science. Last semester, I had a heck of a time in my circuits class when I needed to use complex matrices, which is only supported in the TI-85 and higher. I ended up borrowing my roommate's TI-89 for the tests.
Most people probably won't need this function, so the 83+ should be fine. I just got tired of solving down systems of 4-7 simultaneous equations using Cramer's rule.