How to price commissions?

Hi all, I had someone reach out to commission an urban-sketching style illustration of their business (small building) to put up on their website.

I have no clue on pricing. How would you go about it? I want to charge something reasonable, I don’t want to lose the opportunity because it’s something very exciting for me, but I also don’t want to undersell myself.

Some info: this is in the USA, I’d use watercolors and fineliners. I do need to buy decent watercolor paper if I do this.

Just a note that this would be using photos as a reference, so not on site.