I believe it's cheaper to purchase a new external bearing set than it is to purchase an entire new internal bearing BB. Of course there are internal bearing BB's that are said to be rebuildable. I owned a couple high end Race Face BB's, and they claimed they were rebuildable. But I had no luck ordering the special tool and bearing kit to rebuild the things. The shops kept telling me they simply throw them away and install a completely new unit. I guess that's the easy route when you're spending someone else's cash.
There are folks here on the forums who say that it shouldn't be a problem to buy the bearings and install them in high end, internal bearing BB's, and that they will be doing just that when the time comes. Then again, I've never read here of anyone actually doing it.
I've gone to buying mid-priced BB's for my current bikes. They seem to last as long as the more expensive ones. So all things being equal, they make for far more attractive 'throw-aways'. If and when I build another new bike, I'll probably go with the external bearing type BB.
DanO