Commentaries:
Essentially, David is saying that the offering of an animal does not meet the price of forgiveness, but an acceptable sacrifice before God is a broken and contrite heart. An acceptable sacrifice is repentance, as it is giving up human nature, its own will, and obstinacy and pride. They have been suppressed and then replaced by humility. This is personally costly because it motivates one to submit his life to God.
John W. Ritenbaugh
Preparing to Be a Priest
When David saw the enormity of his sin, he realized he had hurt God and His purpose. His sorrow, chagrin, and remorse reached deeply into his heart, mind, and entire being. Our opposition to God should create a similar deep emotional response in us, for we have all played major roles in our Savior's death. He died for our sins. Emotional sorrow alone is not the answer, however. Paul says godly sorrow produces repentance (change) toward salvation, while worldly sorrow is like saying, "I'm sorry I got caught. I'll be more careful next time I sin."
Martin G. Collins
Basic Doctrines: Repentance