Ash HN: Given the salary of a developer, how can a side project be worth it?

Disclaimer, this is a real first world-problem. As a JavaScript contractor in the UK, I can easily make 500£+ a day, full time, minus taxes. My goal was to reimburse my student loans (done) and start putting money aside for making my own startup. But the more I stay in this business, the more I make money, and the more it's hard to even imagine a startup that comes close to that opportunity cost.

Even for a side project, it would simply be more profitable if I just spent time finding higher paying customers for my JavaScript skills.

It sort of feels like a trap. Anyone got a story of how they went through that?