That strikes me like "Dating advice: Explain to the average woman that accompanying you to the average restaurant followed by an average movie would make for an average evening."

Yes, but to explain why we're worth X% more than the average developer/freelancer, I need to know how much they make in the first place!

I'm pretty sure you don't. In fact, I'd go so far as to say, framing it terms of another developer will hurt you if you are charging more.

If you are delivering $100,000 in business value, and you charge $20,000 for it it is a win for the company. If another developer charges $10,000 for it. Going with them isn't twice the win for the company.

You don't say I'm worth x% more because I work x% faster. You are worth $20,000 (which just so happens to be x% more) because you've done this before, helped other clients realize 5x ROI and you've worked around the nasty #2343 bug in the database that will bite them if they go with someone else.

In my experience this is a bad way to approach things. Far better to frame the conversation in terms of how much value the customer will derive.

My personal example: through a combination of political cleverness and tech-savvy, I saved a company several million dollars. That anchor makes me seem much more valuable than if I start by comparing myself to an average programmer, and describe how my services are better (in fact, my technological skills are substantially WORSE than the average programmer's, which would make it an even harder sell).