I'm now a developer, where to go from here if I want more money?

By getting promoted up the developer ladder - lead developer, development manager, etc. - you end up in positions where you have more influence on the products the company builds, and that usually comes with a higher salary. The downside, at least for people like me who would rather code than manage, is that it also comes with the responsibility to manage other peoples' work, and eventually to manage personnel issues like hiring and firing.

This is true in all companies, not just startups - every company needs some number of people in leadership roles. A possible advantage of working for a startup is that you might be given broader responsibilities a bit sooner.

"Startups pay very well."

The consensus I observe on HN is that startups pay developers significantly less than large companies like Google or Facebook, but try to convince employees that the tiny percentage of equity they're offered (which is most commonly worthless, since most startups fail) makes up for a decreased salary.