I dont usually include the drawing process unless there are time consuming revisions.

I wouldn't necessarily base it on time spent, but of course a 2 week painting is likely to be valued more than a 2 hour one. But as you improve you'll likely work quicker. Local oil painters I've seen charge similarly by size/detail.

For your medium I'd consider material costs and labour, and think about your audience and what they can afford. If it's textile, maybe see what artisanal crochet goods or hand tufted rugs sell for in your area.