I don't get how a cunning Slytherin like him can discard such an advantage for pure vengeance over an "attempt" which didn't do any real harm.

On the contrary:

And Draco was getting angry again. "Dumbledore killed Mother, it's not enough to just say it's sad! I don't understand what you think you have to do, but the Malfoys have to take revenge!" Not avenging the deaths of family went beyond weakness, beyond dishonor, you might as well not exist.

Lucius never meant for Harry to accept the monetary bargain. This is clear from his reaction. He wanted his revenge, and Harry was interfering.