Ask for as much as you think the potential patron can pay. If your quote is turned down, the don’t take the commission. In any case, what you ask for will be a pittance of the true value of the work. No one will ever pay you what your work is actually worth unless you’re building models for a corporation that provides that sort of service to other industries.
Scale model making is skilled, precision work that takes talent that has been developed over years of practice. If you spend 40 hours building a largely OOTB kit and charge $400 for the commission, you’re making less money per hour than the kid flipping burgers in your local fast-food joint.
Speaking for myself, I very rarely ever take on commissions. At one time in the past, I considered even being asked a great honor and usually jumped right on any solicitation to do one. However, I quickly learned that: 1. No one was willing to pay anything close to a reasonable wage for the work, and 2. Building models for other people sucked all of the fun out of it. Commissions are WORK, not hobby.
So, since commission builds are work, but no one is willing to pay a fair price for that work, then doing commissions is simply punishing yourself.
Nowadays, if I am solicited for a commission, my only real consideration is MY OWN INTEREST in the proposed build. Unless it is something that I would like to do for my own reasons, I will turn the job down. If, on the other hand, it is a subject that I’m interested in doing, then I’m willing to accept whatever the potential patron will pay. The money becomes absolutely a secondary consideration.
In such cases, I ask for the world but will usually accept much less - as long as I will have ZERO out of pocket costs in the end.
To manage my costs, I also keep track of my expenses and provide the patron an itemized invoice for those costs which is negotiated to be paid separately, over and above the commission rate for the model. If the potential patron will not trust me to be honest with my itemized invoice for materials and consumables and will not agree to pay costs for those OVER the base commission rate, then I will also not accept the job.
Quoting the job is then done in two parts: the base cost for the build + the reimbursement for my actual out of pocket costs (TBD) = total due on delivery.