What do other companies say? "Nah you're not" is a pretty standard response to "I'm worth more than this" but consider the source. For some reason many companies would far rather let a good employee leave than raise their pay to market rate.

> Implying that other developers in my area are available for less than I ask.

They can attempt to prove it.

I had a buddy do exactly that. His manager told him he was already paid above market. Inside a month he showed her that she was wrong and we had a gaping hole to fill in the team.