Opteron is AMD's server processor. Unless you're interested in server processors, you're probably referring ClawHammer, which is exactly the same as Opteron minus features like dual channel memory, 512K instead of 1024K L2 cache, etc.
If I had to take a guess, I'd say that the first ClawHammer processors will run between $800-$900 (assuming they outperform the fastest Pentium 4 on the market at that time).