I believe it's meaning can be found in the scene between Billie and Frank, where Frank draws on the table. Note that it is a picture of rats, swarming over the MA state dome. Why? Because I believe that the Gold Dome represents the American ideals of ambition, power and above all, honesty. To be golden, as it were. However, as we see throughout the film, the actions and decisions required to make it to the gold, are not so pure. It's all an illusion, at least, by Frank's perception. Billie's too, by the end of the film he mocks "the state funeral, with all the bagpipes and shit".
He does this, because he understands that it's all just for show, and that only rats make it to the top. This can go further with the rat motif, because notice at the end, a rat crawls across Sullivan's balcony. At last, he had reached the top, but in the end...he was only a rat.