How much should I charge for this?

“How I’m Feeling Now, Charli XCX” 19 x 24” Graphite

This took about 25 hours, give or take. I posted the progress on my snap story and someone asked if they can buy it. They live waaay out of state so I’d need to ship it to them but it saves me from buying a frame for it.

It’s hard to photograph graphite and it’s even harder to decide how much to charge for work. I’m trying to step it up in a creative field and I need some advice on how to price my work…I usually charge like $10/hour plus materials or shipping costs…help!