Are there any reasonably priced books or scholarly articles you would recommend for someone who wants to learn about the chthonic gods and the greek underworld as a whole?

The Greek Way of Death by Robert Garland.
About funerary rites and the ancient Greeks’ attitudes toward death and the dead. Includes the chapter “Life in Hades” that gives a short presentation of the underworld and the divinities that were believed to reside there.

Restless Dead: Encounters between the Living and the Dead in Ancient Greece by Sarah Iles Johnston.
This book discusses how Greek ideas about the dead evolved during the archaic and classical periods. It also includes a chapter about the development of Hekate into a mistress of ghosts, and the nature and function of the Erinyes.

“Hades: Cornucopiae, Fertility and Death” by Diana Burton.
A small text that discusses how Hades/Plouton appears in ancient Greek art. Can be found here.

These were the ones I could think of that are not so expensive (unfortunately, academic books often are quite expensive). Maybe @coloricioso  or @disbander-of-armies would know some more?