So, you need to design the PCB. You need to get artwork made. You need some prototypes. Then you need to decide whether you'll re-design, or include a mod. Re-design is expensive, that is balanced by having a bit of wire soldered between two points and a track cut by a low paid operative in some random factory.

You now have some PCBs. You need production aids. You may need stencils for screen printing solder paste. The design can be generated from the PCB gerber files. But again, there are artwork costs for this. You need a pick and place machine program. If there are many conventional components you'll want a cropping machine plate made. (A steel plate which the PCB sits on with small holes for conventional components; you then put this on the cropping machine which pins the components down while a blade slides and crops the component leads).

Then you build your prototypes. You need someone to buy components; to put these onto the pick and place machine (Which means stripping that machine down from the last job); to load the programs and set the reflow oven and align the screen print stencil.

You can new see that it's just as easy to produce ten as it is to produce one. (I'd recommend people building prototypes to build a batch of, say, 3 rather than just 1 or 2. That gives you an early alpha; a beta, and a neat and tidy beta to create documents and production drawings from.)

Hopefully I've shown how many of the costs are up front. The costs of components and actually building stuff is relatively cheap.