First contingency client wants a flat fee instead — how do you redirect?

First real prospect after finishing the AAUBA course. Manufacturer, ~$45K/month combined utilities. He's saying 'just charge me a flat $5K to do the audit.' Contingency works out to 35% of identified savings which on his volume is likely $20-40K. How do you frame this without sounding like you're trying to maximize your own pay? Want him to understand the alignment but not over-explain.