CNC boards sound like a great idea, but except for the better drilling ability of a CNC, etching probably makes better boards and is way cheaper.
The thing is either way, you will put a lot of time into perfecting a method, and then as soon as you use a PCB service like JLCPCB, you will stop making your own boards. Even if you can make your CNC for $250 (you can probably do a 3018 Pro CNC for that), you will probably end up spending more on drill bits, router bits, collets for the ER11 chuck, offline control panel.
The PCB services give you very low cost single, double, 4 layer etc board. You get solder masks, silkscreen layer. Many of these services provide you with 5 or 10 boards for an amazingly low amount of money. You can use very fine geometries and you need that for some modern SMD parts. The solder masks mean you can solder SMD parts without solder bridging. The copper is plated and you can get gold plating if you need it. You will end up with professional looking prototypes rather then oxidising copper tracks on a rough PCB.
I would only think about the expense of a CNC solution if you had a particular reason to need to build simple PCBs on a very short timescale. If you have to order components, you probably can get the boards back from the PCB services before all the components arrive.
Richard