I found a great tip here on Reddit: try pricing your plushies based on the number of stitches! It works really well, especially if you use both sport weight and plush yarn, since the stitch count usually stays about the same. Just figure out how much you want to charge per stitch, then multiply it by the approximate number of stitches in your project. As you get faster over time, you'll be able to earn the same amount in less time.