The origins of this 9.99 price point are disputed. Some allude to the “charm pricing” technique purporting that bills ending in 99 feel more than one cent cheaper than the next highest integer. Others say the number was set to mirror the cost of a Blockbuster rental card. Whatever its true genesis, the two-decades old 9.99 price point is the missing note for the ongoing chorus of debate about how much music is worth and what it should cost.
Source: Malbeconomics: Taking stock of the twentieth anniversary of the 9.99 price point — Tarzan Economics