Brains

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!''