I always try to be honest to myself so I don't end up working hard for nothing. First I asked if at all possible for them to supply the fabrics. This gives the customer an appreciation for the cost of the materials alone. I went to The City Quilter and came out with no fabrics after seen what fabric is going for.
I then think how much I would be willing to pay for it. I know the effort, time and expertise required to come up with a piece like that, and the amount I would be willing to pay for it is never low. Also, if you feel resistance, remind them that due to the nature of the quilt you will be making, they are getting a piece that will be impossible to reproduce. Copy, maybe. But never reproduced in its entirely. On top of that they want pillow shams, I say $1500 or no quilt for you. I have sold quilts much simpler for close to that.
Good luck. Your quilt is amazing!
One question...Did you applique the leaves after the complete background was pieced? For that alone I would charge even more. That is not easy to do.
Maria