Over the past 10-12 years I've built and sold a few models. Primarily to folk who wished to give them to a relative who had either served aboard a particular sub or ship or who had flown or worked on a plane or chopper. One of the last ones I did was of a TN ANG RF-84F which had been purchased by an individual who had flown them back in the '50s and had retired as a Maj. Gen. in charge of the TN ANG. He is up in years and I could see that he would not be able to complete the kit and have it look like much, Parkinson's is setting in. He is a good friend and I did it for nothing. Fortunately, he had pics of his personal bird and I was able to print out decals to replicate it.
For those models I have been contracted to build, as a rule it all depends upon 1) cost of the model, 2) complexity of the build - straight out of the box or a personal bird/ship/sub where unique decals have to be made, 3) whether a diorama is involved, 4) what kind of base is desired, and 5) whether a acrylic case to cover the subject is wanted. I usually throw in a brass plaque describing the subject and the name of the recipient which I have a local trophy shop make. As a rule of thumb I have charged anywhere from $100 up. I was paid $1000 for the most expensive one I did. That one involved a CH-47A in a fairly large diorama and took nearly a year to complete. (Around 1500 1/48th scale sandbags were involved.) Once I got it done, and delivered, I broke out the calculator and figuring I worked a minimum two hours a day, five days a week on it, I came out making under a $1.00 an hour.
There is some regret in parting with a model which you have spent a bit of time on, but it is outweighed by the pleasure and awe expressed by the recipient. I've had customers choke up and have tears flow when I deliver a model of a subject they served on or flew. Cases like that give me a lot of job satisfaction especially when I know that the model I've built up will be handed down from one generation to another.