Ok, I have to start right off letting you know that this post will not reveal a formula for pricing your work! The fact is, there is no formula. Even though I've been creating commissioned mosaic for about 2 decades, I still don't have a solid equation for mosaic projects.
When I was just getting started, I remember being very confused about how to price my work. I just made up numbers by guessing how long it would take me and how much materials would cost, and I was working for peanuts. Honestly, that is how you start. If you are like me, you are getting started with very basic skills and each project poses new and unanticipated challenges. You are making mistakes and learning as you go. Price accordingly and be honest with yourself. Do as many projects as possible in your own home or as favors for friends in order to learn and build your portfolio before accepting commissions. I can't stress enough how important this is! If you take on a corporate or public art project before you understand the properties of adhesives and how they combine with different substrates, you are setting yourself up for a disaster.