2 Samuel 24:24 (NKJV)
Then the king said to Araunah, "No, but I will surely buy it from you for a price; nor will I offer burnt offerings to the Lord my God with that which costs me nothing." So David bought the threshing floor and the oxen for fifty shekels of silver.
King David wanted to purchase a piece of property which was to be the future location of the temple in Jerusalem. The seller wanted to just give it to him, but David insisted on paying a fair price for it. David thought it would be an insult to offer to God that which did not cost him anything. He knew that it is the giving up which made the sacrifice, and not the sacrifice itself. This is why when Jesus saw the widow put her mite into the offering he praised her for her sacrifice. When we give, we should remember that God does not look at the amount on the check but rather what is left on the stub. Are you giving sacrificially?