To use another Castlevania example, Castlevania Chronicles: Cipriani found a three-year high price of $70 for the 2001 PlayStation release in 2007. Two years later, after the game's release on PlayStation Network, physical copies dropped to a low price of $26.93. The people who just wanted to play the game could now get it for six bucks, leaving the PS1 discs for collectors only.
It's not as simple as a re-release dropping the value globally, however: the data shows some weirdly fluctuating prices, like Persona 2: Eternal Punishment on PlayStation, which seems to have gone up in price significantly following a reissue. Perhaps interest rose when buyers got the idea that it was even possible to buy an old Atlus game.