Your cost analysis will put the custom rig well above the cost of a consumer drone and the finished appearance won’t be as clean and appealing.
To build something capable of effectively carry the “good stuff” will have you spending >$2,500.00 just for the basic platform. Cameras, lenses, gimbals, payload controls, and video transmission equipment add considerably more. If you elect to build one don’t do your shopping at Hobbyking, Banggood, or other suppliers of cut rate hobby equipment. You can’t afford that level of unreliability.
The cost is what inhibits most people. Size is also a limiting factor as they have to be larger to carry a heavier payload. Size reduction was one of the reasons X-8’s and Y-6’s were created.