First off, prices that low are loss leaders, designed to get you into the store and buy other stuff. Next, remember that corned beef contains around 35% solution added. So a 5lb corned beef is really only about 3.25 lbs of meat.
As for why the OP can't find lean flats, I have no idea. Around here, flats are usually trimmed of all the fat cap and have hardly any internal fat. It sounds like what you are seeing is not a true flat, but a brisket cut in half so it has part of the point attached to the flat and has the layer of fat running between. I've never seen them sold commercially like that.
I'd suggest getting a full packer and separating it yourself. You'll end up with a lean flat you can use for corned beef. The point can be ground into some of the best burgers, or use for burnt ends.