Original retail price of the 8-core was $3,299. Original retail price of the 3.33GHz quad-core was $3,699. The 8-core uses two $375 processors, the 4-core a single $1000 processor. The 4-core has a bigger premium as it was an upgrade to get that rather than being a base-model as the 8-core was. The current 8-core model is cheaper than the 6-core, despite coming with twice as much memory and a more complex logic board as the processor prices are the same as the models we are discussing. There is nothing weird about the prices of the two models you showed. One is discounted 18% the other 15%.
More cores can mean more power, but the software has to take advantage of it. There is also some overhead when using many cores. Most people should be looking to get the fastest clock speed they can in quad-core when it comes to desktops for serious use.