And Gad came that day to David and said to him, "Go up, raise an altar to the LORD on the threshing floor of Araunah the Jebusite." So David went up at Gad's word, as the LORD commanded. And when Araunah looked down, he saw the king and his servants coming on toward him. And Araunah went out and paid homage to the king with his face to the ground. And Araunah said, "Why has my lord the king come to his servant?" David said, "To buy the threshing floor from you, in order to build an altar to the LORD, that the plague may be averted from the people." Then Araunah said to David, "Let my lord the king take and offer up what seems good to him. Here are the oxen for the burnt offering and the threshing sledges and the yokes of the oxen for the wood. All this, O king, Araunah gives to the king." And Araunah said to the king, "May the LORD your God accept you." But the king said to Araunah, "No, but I will buy it from you for a price. I will not offer burnt offerings to the LORD my God that cost me nothing." So David bought the threshing floor and the oxen for fifty shekels of silver. And David built there an altar to the LORD and offered burnt offerings and peace offerings. So the LORD responded to the plea for the land, and the plague was averted from Israel. (2 Samuel 24:18-25, ESV)
If I pass along a gift to you that I happened to receive from somebody else, nobody is really harmed in the transaction. If you happened to want what you received and I didn't have much use for it, you might even say that you came away satisfied. But if I say I've thought long and hard about you when I selected some special gift - that I sacrificed because I knew you would deeply appreciate it - and you later find out that I simply re-gifted an item I didn't want, I would imagine you might think a bit less of me or, at the very least, my integrity.
Contrast David's sacrifice here with the one Saul said he intended to make as recorded in 1 Samuel 15 and I think you will begin to see the similar picture to the second example above. Saul remained arrogant and conniving right to the end (need I bring up the whole witch at Endor thing). David, when presented with a legitimate offer from a loyal subject flat out refused to take the cheap way out. He was the one who had led the transgression (counting the fighting men) and he realized he needed to lead the act of repentance. He would not offer up some second-hand sacrifice to the Holy God.
In my current Lenten reflections along the Crooked Path, I realize that desiring to deepen the relationship with my God is a good thing. And, in doing so, I need to give back to Him things that I hold of value. The giving is not so I can gain (at least not as we usually count gaining), but rather because of what He gave up for me. In the face of His sacrifice of Jesus, how could I possibly offer God something in which I have no stake? A true sacrifice of the heart comes at a cost - a personal cost. Nothing else can substitute for that.