Easy or difficult is a relative thing. Someone with basic machine skills can complete a AR lower on a manual mill. On some sites I visit I have seen AR lowers finished and working with nothing but a drill press dremel tool and files. CNC can help if you are making 5-5000 of them or carving the whole thing from a rectangle of aluminum.
If you are in manufacturing this project is pretty simple. Custom fixtures are minimal, no special tools, 7075 aluminum is relatively easy to cut, the overall accuracy is not to demanding. On a scale of 1-10 with 10 being most difficult I give a project like this a 3-4. I'd give it a 2-3 if you outsource the magazine well. The mag well is the most difficult feature because it is a narrow and deep with sharp inside corners.
Here is an example of a great tutorial with manual machines starting with a 0%:
http://www.ray-vin.com/frtech.htm <--------The link is in the center of the top row on this page to start the tutorial.
It is correct that making your own is not a way to save money. The people going to the trouble do it for the sake of doing it or to make something that can't be bought off the shelf.
The only thing to remember if you are making your own guns, they MUST be legal to own in your area, they MUST be for personal use only and CANNOT be made with the intention of gifting or selling.
If you make one be sure to post pics!
