An art supply company would like to place an add on my website and likes to pay for it. Any idea how much I should ask for?

I have no idea but what an honour congrats

It depends how much visitors your sites has. But ask for at 100USD/month and then start negotiating.

I probably can't backup $100/month with my page views, but I shut not ask for to low because they probably will negotiate.

Thank you both for your support!

you could let them make the first offer and go from there.