I solved a test problem for a company once (gt 500 LOC), but it was after a pre-screen and the stated salary was 130-140k base, not including generous bonuses, in the Midwest. It was also when I had a lot of downtime, so I didn't mind.

Now, I have a hard time finding enough time for my own projects, much less for contributing something to a company just for the chance to interview.

Since you're offering single digit equity, it might be worthwhile, but for the vast majority of companies, it would warrant a pass.