Answer:
Your answer would be that the adjectival phrase is of the new bridge.
Explanation:
In the sentence above, the adjectival phrase is of the new bridge. This is a multiword adjective which is not headed by an adjective. The phrase is headed by the preposition of. However, this string of words acts as an adjective because it is modifying a noun, the NP the cost. That is why it can be considered an adjectival phrase.
