Let me put it hypothetically to illustrate a point. (I am not a pro, and I do not really know the "real worth"). Let me assume that the real worth of the job is $1000.00 (reasonable assumption?}
So you are doing him a favor by telling him that he should not be exploited by taking only $200.00. So he listens to you, and everybody else too. Nobody accepts the $200.00. Everybody insists on $1000.00. The client has no choice but to up the offer to $1000.00.
Now who will get the job? Will you, assuming hypothetically you are an establised pro, get the job? Or will you, with overflowing milk of human kindness, offer the job to the guy who turned down the $200.00 on your advice? Or would the client pay this guy $1000.00 anyway?
Have you really helped him? Or had you, in your eagerness to help him from being exploited, actually did him a disservice?