Ask HN: What pay to target when joining a startup?

Can anyone recommend advice to a senior engineer at a big tech company who is looking to join a startup as employee no. 3?

Let's assume the company is just kicking off and has secured $600k - $1m of initial funding. How do you think about pay? What are good questions to ask before committing?

I'm only used to negotiating with a large corporation that is both public and willing to show actual money.