There is one other reason I find externals cheaper at times, especially when on sale. Product changes. They come out with newer models of external drives more often than internals, so put up the old external models on sale to clear them out.
I have gotten some good deals by removing a drive from an enclosure vs buying the same size drive as a bare drive.
Keep in mind that another thing is that many external drives are slower 5400 rpm and older models as well since they go more by size and enclosure looks than drive performance.