I don't do prop commissions, but for sewing commissions the way I price myself is time + materials used. I give myself an hourly rate and include the price of material's when it comes to quoting someone.
I would suggest to price yourself fairly. I understand sometimes giving a quote can be intimidating because the price might seem high, but you're doing custom work, and you should be paid accordingly for it. You don't want to price yourself too low that you're going to be taking a cut on making something. I would suggest getting a time tracking app so you understand how much time you're putting into working on something. You don't realize the amount of time spent working on a project until you time it out.