Ray your thoughts are right on. It is cost to some extent, for us it was, and if I remember correctly the internal sensors are north of 1000 bucks, not including the toad. The Pressure-Pro is about 700 and that is including the toad.
However you will not have concerns about someone (kids, I had a couple lifted from our P/U) removing them. You can air the tire up normally without having to remove a sensor, do not have concerns about valve stem leaks and the internal ones also give a temp. that the external do not.
So it is not like you are not getting anything for the extra cash. Add the expense of mounting and dismounting tires unless of course you coordinate the purchase with new tires.
I would buy Pressure-Pro again but given some extra cash I would love to upgrade to the internal style. I'm guessing that the internal ones have a battery life issue too, I do not see how they would function without one.