This is a difficult question, I build aircraft for a fellow aviation enthusiast and charge between £100 stirling (180 dollars US) and £150 per completed kit in 1/72 scale the price increases for larger or more complex kits, the first being a virtually scratch built kit that netted me about £300. The guy that I build for understands the amount of work involved and the fact that the finished product is unique unlike a £25.00 die cast (many of which are nonetheless excellent). At those rates I gain on the simple kits and lose on the more complex ones.
Even a straight assemble and paint job on for example the new Tamiya Impreza WRC (A flawless product) is going to take quite a few hours. What is your hour worth to you?
If you do build for someone else chances are you will someday be asked to build a kit (subject) that you have no enthusiasm for and then it becomes hard to stay focused and do your best work. I am lucky in that respect, my clients tastes (in aircraft at any rate) are similar to my own.
I hope that this is of some help to you
How do you make a million in the modelling business? Start of with 2 million and know when to quit!