I work as a freelance teaching artist with a focus on integrating art and science. My most popular workshop is in nature journaling, which I can adapt to be either indoor or outdoor and from one hour to six hours. I've run workshops for organizations as diverse as Paul Smith's College and the Green Mountain Club. I recently joined the Vermont Teaching Artist roster on a provisional basis, which means I am eligible for the Arts Council's Artists in Schools grants.
Standard teaching rate: 40$/hr in-workshop time
This covers standard prep time, use of my collection of art materials, my natural history teaching collection, and my science equipment (mostly microscopes of various kinds, some other lab equipment). Long and involved prep time and additional or special materials would need to be negotiated - I would let the school know upfront if any proposed plans fell into this category.
Travel over 30 mi from Duxbury reimbursed at 55c/mi or the rate used by the school if they have one.
For nonprofits, I use a model where we agree on a class per person fee based the organizations existing fee schedule, which usually takes into account member and nonmember rates. I and the nonprofit then split the proceeds of the workshop.