It's unsurprising that cheap and readily available digital downloads for rare games decrease the value of their physical counterparts -- some owners of unreleased titles and prototypes guard their game's worth by refusing to make its ROMs available to the public -- but retro gaming site Racketboy actually tracked the effect of PSN/XBLA releases on collectible titles.

In the above chart, you can see the drops in pricing across all Marvel vs Capcom 2 retail copies when the PSN and XBLA porters were rumored, announced, and finally released. At its peak, the PS2 edition of MvC2 was valued as high as $110 (according to an average of closing auction prices on eBay for that month), dropping to almost $40 four months after the digital download was made available.

Racketboy posted similar charts for Rez and Ikaruga along with several useful tips for collectors like "If you aren’t especially attached to the game, you might want to sell on a strong rumor of re-release," and "Prices won’t bottom out completely until it is actually re-released."