Hi Sarah,
I would start by re-evaluating previous advertising campaigns. There must be a reason why they “did not work”. Check the usual suspects: when the campaigns were done, in which media, did anyone actually measure the response (and I seriously doubt this

) and how was it measured.
This should highlight critical and problematic areas. Then do some prognosis on ROI.
Afterwards, just go and speak with your boss, saying – I know how we I can drive x number of customers to your business and it will cost you only x amount per head, taking under consideration that our margin is x %, we should be making x amount in profit.
He would be mad, to refuse funds then, wouldn’t he?
Good luck
Fili