Zero.
Value the equity at zero.
Or at most value it like a lottery ticket - something that lets you amuse yourself dreaming about what you’ll do when you “win”.
But make all your ongoing plans assuming they equity will be worth zero. Either because the startup fails, or because you get screwed out of any payout by the founders or board, or by future investors insisting on deals where early employee equity gets diluted and preferenced down to zero. Or even occasionally because they intentionally never have a liquidity event and stay private with no available way to monetise your shares. Those, or exits where employees make at most “new car” money not “fuck you” money, are by far the most common.
Do a startup if you want, they can be amazing ways to gain an incredible amount of experience to add to your resume in a short time, they can be a great fun work style at least for a while, they can be great at finding out who amongst your co workers are genuinely super talented and productive people and building networks with them.
But don’t fall for the “yeah, we’re paying well below market, but look at how generous our option grants are!” snake oil recruitment pitches…