Well one of the things you'll likely face is that the client will leave your services for that of another domain registrar, as they likely DO offer it for free.
Years ago, it was a big racket - up to $10/month for privacy protection. Then a few places dropped the price to $10/year. These days, you can find plenty of places that offer it for free. There's any number of domain reseller companies that also offer it for free, so you can take advantage of that there too if you wanted.
I guess I compare it to the people that are still charging $14.95/year for domains, or $24.95/year - and they're just regular .com domains. It's not a competitive price, but if their clients are paying it, then that's all that matters in their eyes. I have a different view of things, but everyone has their own opinions.