You also have to look at the prices of competing products. Consider the 40-pin Basic Stamp, at $80+.
In general, small volume electronics sell at about 5x parts cost (you can find discussion at various places.) That makes the Mega pretty cheap, and the duemilanove VERY inexpensive (perhaps it has passed the "small volume" mark!)
(Of course, the nice part about the regular arduino is you can build your own, substituting this and that, and get something working for even less outlay of $$, which is harder to do with a board based on a large SMT part like the Mega...)