。
She agreed to pay half of the rent.
Literal
She [topic-は] rent [of-の] half [object-を] pay [for-のに] agreed.
Two pieces deserve a closer look. The のに here is the purpose-marking のに (V-plain + のに = 'for the purpose of V-ing'), distinct from the despite-marking のに you encounter in ~のに ('even though'). With verbs like 必要だ, 役立つ, 使う, 払う, this purpose のに is the natural choice. The verb 同意する ('agree, consent') takes its object marker with に, just like 応じる, 賛成する, and other verbs of accord. Together: 'agree to (the act of) paying half the rent.' Common in roommate or housemate negotiations.