At one end, there's making one-offs or small editions that sell for thousands of dollars each (or hundreds each for larger handmade editions). In terms of physical production, a binder could make hundreds from scratch every year; in practice, I believe the market for a binder's artistic works is smaller than that, and everyone I've met supplements that with teaching, grant money, restoration, or ancillary work (such as clamshell boxes or custom rebinding). The person with whom I'm apprenticing makes a good living at it, but he concentrates mainly on restoration and ancillary products, and has a strong side business selling rare books (partly because he can buy old but poor quality books, restore them, and sell them for much more).
It's a business very dependent upon networking within a collector community, and I have no idea how large it is, but it's definitely there and has deep pockets. One student I took a class with made a decent living making clamshell boxes for collectibles, and one collector brought her an original script from Star Wars, signed by the cast, including all the verification paperwork, and one of the prop light sabres.
I don't need to make a living at it, so I'm lucky. But it's definitely an area of the arts with arts level compensation, which is to say... not great.