The prince's thought shows that it is not possible to pay a life with another life, that is, he shows that killing Romeo will not pay the debt for Mercutio's death, besides that whoever kills Romeo, will have the same debt as him.
Explanation:
When the prince asks "Who now the price of his dear blood doth owe?" he is showing that Mercutio's life is priceless, nothing will make Romeo pay for this life and neither Romeo nor anyone can bring Mercutio back, that is, it is useless to kill Romeo and turn another citizen of the city into a murderer. Therefore, the best punishment is to exile Romeo and let him suffer the consequences of his own actions.
