In tech terms, miniaturization costs are ever decreasing, and the complexity and capabilities of tinier devices are increasing. It will not be expensive when deployed in 1-2 decades. Just to put things into perspective, the capabilities of a modern smartphone exceed those of a multi-room supercomputer 20 years ago or a desktop computer 7-10 years ago.

So, to answer your first question, it's mostly "speculation" based on trend extrapolation.

In terms of references there is an entire field called nanomedicine*... https://en.wikipedia.org/wiki/Nanomedicine

...which is researching nanotech based applications in medicine.

"Nanomedicine sales reached $16 billion in 2015, with a minimum of $3.8 billion in nanotechnology R&D being invested every year."
...
"Another vision is based on small electromechanical systems; nanoelectromechanical systems are being investigated for the active release of drugs."