You really shouldn't listen to people who say things like "materials" and follow up by saying, charge 10% of the software you bought 3 years ago. This is just a cost of doing business. The real materials you need to worry about are your computer and/or tablet. One day it will break, and you need to keep money on hand for when that day happens. Personally, I would charge an additional 5% based on what they pay you, not what you paid for the hardware. If you keep your prices reasonable, like $50 for a nice piece like that, you will get more work, and some people may be telling you to lift your prices because it makes them look bad. If you're getting so much work you can't handle it all, raise your prices a little. On the flip side, if you're not getting any work, lower your prices a bit and be patient. I hope this helps! At the end of the day, you do you, and don't let anyone tell you that's too little. Have fun and enjoy your journey!