I’m having a problem with this as well. It can get really expensive sometimes purchasing materials and therefore, most artists don’t include the right amount for the time spent on the piece. I think that it all depends on how well you are established. Maybe charge less, at first, to get your name out there and then start charging more once people know your work.
When it comes to crafting, it all depends on what you’re doing. Crocheted items can be very difficult to make a decent amount for your time. I crochet and I will tell you that I cannot get anything approaching what my time is worth when selling thread centerpieces or larger projects.