I once read that beyond $4k for speakers, you were buying the bottom few Hz and the top several db. That has always seemed about right to me - there are many speakers in the <$4k range that to things REALLY well but are not the last word in bottom end extension, and would not necessarily fill a large room or play AC/DC at concert levels. Examples that come quickly to mind are the Dunlavy SC-III and Dynaudio Contour 1.8. There are many others to be sure.
I think it is also safe to say that to achieve "the ultimate", you're going to have to spend more. It is not safe to say the reverse - that is, that if you spend more, you'll get closer to the ultimate. -Kirk