The relationship between the supply of substitute goods and relative prices is a fundamental economic truism governed by supply, demand, and market equilibrium principles, requiring no citation.
The claim states that an increase in the supply of a substitute good alters the price difference between substitutes. This is a basic deductive implication of supply and demand curves intersecting: adding supply to one good shifts its price, which directly alters the price differential between it and its substitutes. Because this follows logically by definition and everyday observation of market mechanics, it is categorized as common knowledge (Step 0).