If you can impress upon your customer that you are worth every penny you ask, it's not bad creating art on commision.
Talk politely with the customer, find out what is wanted, you set the deadline, you set the price, come to an agreement. Write it down and each of you keep a copy. You do not apologize for the time it takes to do a good job. Figure your time and add 20% for whatever. If it takes 9 months because there are other orders in house the customer has to accept that. You require a deposit to put the customer in line - first come first serve.
If you don't value yourself and your work, there is a chance the customer won't either. Of course you do good work. :)