Pricing Mosaic Commissions

I guess it’s like any art work we make.  Always hard to figure out how much to charge for a piece.  It’s easier in some ways to price something you’ve made for sale than to price a commissioned mosaic.  It’s always a ‘guesstimate” based on previous experience.  I find it really difficult.  Seems I get into making something and it takes me a lot longer than I thought it might, therefore, I’m not asking what I need to, to make it profitable.  I’m hoping that with time and more experience, I’ll be able to resolve this dilemma and be much more comfortable asking for what I need.  If anybody has any suggestions regarding this, I’d love to hear from  you.