$100 is the kind of money you might spend on a night out with friends. The fact that $100 could secure a potentially critical business asset should make the decision a no-brainer.
If the client is serious about their domain, $100 for securing the most popular TLD is still a bargain.
If they need to deliberate about whether $100 for the .com is an investment worth making, then they should stop wasting their time (and yours) as they're clearly not serious about their business - which either means that they're idiots or that it's a business not worth taking seriously.