If you consider that the X2 series outperforms a comparable Opteron 2xx Dual CPU system and that AMD has limited CPU making capacity (Compared to Intel) the prices of the X2 are right in line.
Athlon X2 4400+ ($596) > Dual Opteron 248 (2x $445 = $890 ) (Both CPU/Cores are 2.2ghz 1MB Cache)
Athlon X2 4800+ ($1018) > Dual opteron 250 (2x $669 = $1338) (Both CPU/Cores are 2.4ghz 1MB Cache)

I know I for one will be selling my Dual MP 2800+ system along with my A64 3800+ and buying 2 of these bad boys right after they're in stock!