The flat pack ones would be completely finished, all someone would have to do would be put the provided screws in the correct predrilled holes.
My approach is going to be more of a custom one, in that a customer would tell me their ideas and what they want. I'd create a 3d model on the computer to get the design they want and then build it. I also already have all the tools I need due to working in construction. Price and 'salary' from a cage would depend on the complexity the customer wants and size and material. I'd just advise the customer to what I feel is best.
The humidity in the last cage I built stays between 45% and 50% so I mist it, but I know what I did wrong there. The cage is way too tall so there is hardly any humidity at floor level in comparison to the ceiling, which is unfortunately 16" above the snakes head. But as I said, I have learnt from that experience.
I was looking at cages at Repticon this last weekend and I just figured I can build better cages for better prices. It's something I enjoy doing, and if I make a little money doing it, why not?