Why Did They Sell Joseph Into Egyptian Slavery?

In Genesis 37, verse 28, they sell him into slavery. They think that he's being cocky and his brothers become jealous. 20 shekels of silver is about enough money to feed a medium sized family for a couple of weeks. But when you consider for a human life, they sold him for nothing. Joseph was set aside as someone anointed,. as someone special in the plans of God and his brothers grew rageful. And so they sold him into Egyptian slavery.