I'm spending all my videogame-time on Dragon's Dogma and I couldn't resist the opportunity to get the expansion at that price (I'm not surprised at all by the sudden price fluctuation, it happens frequently on Amazon).
By the way, Capcom will grant a few bonuses for customers that already own the regular edition. I remember unlimited ferrystones, a set of armors and something else too (however all the content is listed in the official announcement for the release of Dark Arisen, which seems like a major expansion to me).
After finishing the game (which took me a ton of hours, since I tried to complete every quest available), I'm still playing at it (new game with different arisen and pawn) and I'm sure that I'll complete it another time, with a different approach, so an expansion is extremely welcome for me.
I can't believe that I almost skipped the purchase of this game (I began to play it maybe a month ago, more or less), it's easily my favorite game ever, even if it's far from perfect and it has a few rough spots, but in my opinion the pros are so awesome that they easily outweight the cons: this game is pure fun.
I hope that Capcom will polish this raw gem and deliver an even better sequel to Dragon's Dogma.