A commission is priced in two parts: a making fee for your time, materials and costs, and, if a business will use the work, a licence fee for how they use it. Price it from your usual prices by size, or from your day rate, and use the other as a check. This builds it all into one quote, with a deposit and what happens if they cancel.
The percentages here come from terms other artists and illustrators have published. Your work, reputation and client change the number. For large, public or exclusive commissions, take proper advice before you sign.
Illustrators: the Association of Illustrators (AOI) checks contracts and gives pricing guidance to members.
Visual artists: a-n The Artists Information Company has commissioning contracts, fees guidance and, for members, a legal and tax helpline.
Public art and big commissions: have a solicitor who knows intellectual property read the contract, especially if the commissioner wants to own the copyright.
A guide, not a quote or legal advice. Other Artist Created pricing tools: day rate · licensing.