The statement that a penny bun was not a specific reference to its cost is inaccurate. Originally, a penny bun was a precise and highly regulated definition of what the consumer was purchasing: a penny's worth of grain, based on the prevailing and official price of grain/flour, with a small fee for the baker.
This transparency between price and product was understood by consumers, with serious consequences for bakers who violated it. This price transparency is the origin of the term, and why "penny bun" continues to be used as a unit of economic measure (Gorman, and others). Later, the original term persisted after regulatory price transparency ended, and became a name for a variety of buns, and was no longer a definition of price or value.