The cost is highly dependent on the technology you use. The term "3D printing" is not descriptive enough.

95% of home machines, and cheaper service bureaus use Fused Deposition Modeling (FDM), they squeeze heated plastic through a nozzle and build everything up like with cake icing. There are limits to the material you can use, the orientation of the part and the final level of detail. But the machines are relatively cheap and it you look around you can get things made very cheap.

Other more expensive machines and service bureaus utilize other technologies like Stereo Lithographby (SLA), Selective Laser Sintering (SLS), etc. These technologies have other limitations, but generally offer more detail and are used more often by professional customers. Much much more $$