Two things to note:
1. The Xeon E5405 is a lower performing chip. You're comparing a 2.0 GHz quad-core (Harpertown architecture) to a 2.4 GHz quad-core (Shanghai architecture). A quick google search of comparisons shows that the Opteron 2378 is going to outperform the Xeon 5405. So you're not paying a premium, you're paying for a better chip.
2. Supply and demand is going to vary a lot by region. Note that AMD ultimately does not set retail prices. AMD has never sold directly to consumers. All AMD chips are sold through resellers/distributors/retailers, and are thus set at whatever price the retailer thinks the chip is worth to their customers. You'll note that in North America the price of the Opteron 2378
is lower than the price of a Xeon E5405
. This is why even across different retailers the prices can and will be different.