Why so expensive?
 

An alien walked into a shop and told the owner that he came from Mars and wanted to buy a brain for research.
"How much is this one?" he asked. "Well that one is a monkey brain and it's $20," he explained.
"How much is that one?" he asked "Well that one is a female brain and its $100." he explained.
"And how much is that one?" he asked. "That one is a male's brain and it is $500" he explained.
"Why so expensive?" the alien asked. "Well it has hardly been used!"