Costs is more than just salary. 5 weeks vs 2 weeks of hollidays, 401k, health insurance, etc...
More like 50% more or at most twice as much.
This notion that experienced developers, as a whole in the industry, are making x times more than junior developers is nonsense. Utter nonsense spoken by people who have no idea of what they're talking about. It's true you can find outliers for that kind of thing, and I'm willing to bet they are specialty cases.
Early in my career I would get huge jumps in pay when I proved myself at a project. Almost fifteen years into this career and I don't get those huge jumps in pay anymore after a successful project. Not because I'm not deserving of the praise, but I'm already paid well to deliver based on my experience. A junior succeeds and gets a raise; a senior succeeds and moves on to the next project.
Could I get one of these mythical 300k jobs? I know I can. I'm willing to bet I could snag a 500k job if I pushed myself. But that would require sacrifices I'm not willing to make, give up hobbies that I enjoy, and live in a city I really don't care for. A city that would eat up the extra income anyway.
I'm more than happy with my current job that pays well and I get to mentor juniors every day, which I enjoy.
In SF/NYC, juniors can start under 100k, seniors can go over 300k. Either number could go either way, let's not argue about who got what. The point is, there is a LARGE multiplier.
I recall one friend in London, in a junior position. Left his company because of bad conditions and bad pay. One of his last assignment was to interview some possible replacements. There was one senior guy 20+ years of experience that was great, the company made an offer for around double his own salary.
Do you know what happened? The guy declined.
Guess what we found out later? He joined another company that offered him around three times that double.
Young folks don't realize how expensive experienced folks can be.
So, the only way that is even in the realm of possibility is if the kid was making $30K and the greybeard was making $300K. That's another benefit of greybeards: you get more bang for the buck simply because of the way salaries scale vs. performance that comes with good experience.