Are you including the cost of buying/leasing the equipment, or just the cost of renting time on someone elses?

I imagine CNC mills are easier to rent time on unless you've got a local hackerspace (which quite possibly will not be keen on printing a gun for you). They would also produce a much more practical product, ...and probably be much more expensive to buy outright.

If instead of a CNC mill you just need the equipment to finish an 80% lower receiver, then you need far less money than either of the other two options, and you get a very practical product for your effort.