I dont know where you live in the world but Fry's electronics charges about $60 for a basic building service that also includes installation of the OS.
Fry's and local shops charge $100 at the minimum for the building service that involves checking the stability of the hardware. This includes a CPU, GPU, Memtest86, and HDD test. Temperatures are checkes and all other bases are covered. Also, the extra upcharge is a guarantee the PC is fully ready and stable.
Since you're calling it a CPU build, I'm not sure how much you actually know about this. Are you talking about a CPU only build that runs on integrated graphics?
An experienced builder can piece together a small or a regular sized build in no time. Larger builds that involve the likes of an AIO coolers, doing a good job of routing cables, setting up multiple hard drives, and configuring case fans is time consuming. RGB lights and custom loops will turn the build into an all day project.