To me, there are two ways to interpret this statement.
First, you could take it as a truly sincere statement of regret. Lord Montague is trying to show that he is sad about Juliet dying and he is trying to show respect for and reconciliation with the Capulet family.
Second, you could interpret it as a cynical move on his part. He might be trying to outdo the Capulets. He might be trying to show that he is richer and more magnanimous than Capulet (because he, not Capulet, is the one raising the statue). So if you are inclined to doubt Montague, you can argue that he is doing this to try to show off how rich and how nice he is.