In old times it was not uncommon for a man to provide a sum of money to the father of the woman he loved in order to have her hand in marriage. Many women would be insulted now days at such an idea and would see it as if a man thought she was a piece of property that could be bought, however, then it meant something completely different. It showed that the man was willing to pay a costly price for someone he loved, her.
When God sent his son Christ to die on the cross for mankind, He gave up something very costly, He gave himself. He did so to buy us back from our father Adam, who is representative of the fallen sin nature.