Like the others have said, it is cheaper than the Athlon 64 X2 3800+ because this one is not a Dual Core.
But the REAL reason why this processor is THIS FREAKIN CHEAP is because of Conroe. AMD is dropping their prices on Athlon 64 processors.
But here's the real thing, and I hope you're sitting down because this might give you a heart attack: AMD is going to drop the prices of their Athlon 64 X2 processors by up to 50% on July 23rd, the day Conroe comes out.
You heard right folks, a Athlon 64 X2 3800+ will cost $150 on July 23rd.
Now, Intel's 1.86Ghz Conroe will also cost under $200, and will still be better than a Athlon 64 X2 4800+, so there will still not be any huge reason to go for it.
But, like people are doing with Intel's dirt-cheap Pentium-Ds, they will make for good second systems. Think about it: a Dual Core Athlon 64 processor second system with a 7800GT for around $650. It will be no match for even the slowest Conroe systems, but it will be a very nice computer nonetheless. Just don't use it as your main system, 'kay?