Until recently, I used to work at a company with a fairly comfortable wage, until I got a message from a recruiter. I don't get them too often, and usually they are as good as spam since they rarely offer a better deal.
So, with nothing to loose, I resorted to a template reply: my salary is [20% more than it actually is], can you make a better offer?
Usually, that's the end of negotiations. One day, however, I've got a reply from recruiter making minimal increases to the pay rate, but promising better benefits, more flexible schedule, etc, etc. Meaning they didn't have much more money to spend on wages (it was still relatively high), but were really desperate for labor. So I went for an interview to make sure they weren't bullshitting, and took the job.
Some might say I was in a privileged position, that many people simply don't have any leverage. And you might be right, but if you DO have the leverage, no boss is ever gonna tell you. We have to be shrewd and shameless and push things whenever we can.