I make part of my living building on commission. I've learned the hard way a few things.....
1- a written contract, it can be simple but get all the below in writing.
2- a list of exactly what the customer wants.
3- time frame for the build and always allow yourself some wiggle room because delays do happen.
4- delivery method, in my case you have three coices, pick it up, Fedex next day air special handling, Fedex second day air special handling. Always insure for the full value of the completed model and charge actual cost of packing, shipping and insurance.
5- price, note that it's a good idea to mention here a 50% non-refunadable deposit (a standard requirement for most professional builders).
6- who supplies the kits and aftermarket, if the customer does make it part of the deposit. I prefer to supply everything myself, that way of the guy decides he doesn't want to pay or pay the full price I have leverage I might not have id he supplied the kit.
7- display base style and case cover. I never build on a base without a cover and I charge my actual cost for that (my supplier charges $160 for a destroyer sized case but it's worth it).
I always make sure it's something that at least piques my interest, the customer is a reasonable sort of guy with realistic expectations and the prime criteria is that I have time to build it right. Not knowing your skill level I can't really advise you on pricing, go with something that will make you some money but not so much that the customer expects museum grade workmanship. Don't lowball your bid just to get the job, that can turn into a nightmare fast.
My general price guide is:
planes- 10 times the retail cost of kit and aftermarket.
tanks- 15 times the retail costs.
dioramas- 20 times the retail costs.
ships- 1/350 only, based on plastic or resin (resin gets a cheaper rate since they build mush faster).....I break prices out by class but for example a 1/350 carrier- $2000 basic build with PE rails, ladders, radar, etc. (prewar #5 standard Navy grey, Ms11, 12, 21 or 22); PE catwalks add $250; full airwing add $500; open hangar with interior add $250-400 depending on the ship; light the hangar add $250; sea base add $350; crew figures add $100-300 depending how many you want; Ms12mod or Ms3x scheme add $500.
Am I expensive? Yes, but I do competition/museum grade work and I make sure commission work is built to my highest standards, that is not always the case when I build for myself. I've learned to separate my hobby building from my professional building.