When the prospect keeps asking for data, it is often a pretext to simply put you off. Rather than compare cost of PM vs. cost of repairs on breakdown, focus the conversation on the cost disruption to the customer's operations. How much does it hurt them when they can't ship product or load production materials into a mchine? There is a real cost to downtime in workers sitting idle, delayed billing, reputation, etc.

While PM's never guarantee that a breakdown won't occur, they do decrease the likelihood of unexpected disruptions and allow a manager to control the repair process.