I've added a Commissions page to this website, which outlines process and practical concerns, and a Commissions Base Price Estimator. The Estimator permits a prospective collector to enter the dimensions of a desired commission, and the Estimator will display a base price within specific size ranges.
The Estimator does not account for additional costs, including travel, research, taxes, shipping, etc. However, it does provide a basis for discussion. I had some help in constructing the Estimator from Rapidweaver developers Paul Russam and Joe Workman, and it's based on Joe Workman's Pi Stack. You can check it out on the Commissions page.