How do I know what to charge?

You never actually don't charge a artist/band unless they ask you "How much do you need?" or "How much do you charge?". There are key considerations to look at before or accepting a pick-up/fill-in gig.

As far as "what to charge?". Follow what your heart tells you. If their offer is not what you feel is fair. Ask for what you feel is fair. The only thing they can do is say NO & find someone who will say YES. Just remember it's FEAST or FAMINE. We're ALL in the same boat.

Now a venue is different story.