AMD sells 8-cores starting at $266 (2GHz Opteron 6128) and 12-core Opterons starting at $744 (1.9GHz Opteron 6164 HE). Those are list prices. They are for sale, but not on Newegg except the more expensive ones, so look around a bit and they will be there. Google has a few shops listed:
Cheapest 8-core: http://www.google.com/products?hl=en&q=opteron+6128
Cheapest 12-core: http://www.google.com/products?hl=en&q=opteron+6168
Server boards that support them are relatively cheap, like this one:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813182230
If you need more cores than 12, then you can get 2P and 4P boards for the above, so up to 48 cores. And I believe Intel can do 8P 8-cores (so 64 cores) but for astronomic amounts of money.
As for desktop CPUs, neither Intel or AMD is going above six cores in 2010, but AMD now has some desktop 6-cores for quite cheap. A 2.8GHz 6-core Phenom II X6 1055T is $205 on Newegg:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103851
