From what I have read on the Matrix forum, AE requires you to already have the original WitP. It is an expansion, not a new game. While they haven't announced pricing, the suggestion has been that the expansion will be less than the current selling price of WitP ($70 for digital download, $80 for hard copy).
However, if you've been reading the forums over there, AE is really an expansion in name only. While the basic engine is the same, they have completely overhauled most, if not all, of the details. I've seen companies sell "Gee Whizbang II" with a lot less improvements over the original "Gee Whizbang" than we should see with AE.