The piece of evidence that best supports the theme that dignity has a high cost is "I killed not thee with half so good a will".
By using these words, what Brutus is trying to say is that he was never completely willing to murder Caesar; however, Caesar can rest in peace because he is willing to die now. Brutus dies immediately after saying these words. He had been part of the conspiracy to kill Julius Caesar in order to save Rome. In that way, although Brutus was worthy of honour due to his love for Rome, he had to pay for murdering Caesar with his own death, which reflects the idea that dignity has a high cost.