They are no longer sold through Nintendo, but I paid $34.99 through the Nintendo store (before shipping). These are an "out of print" item, and the only reason they're significant is because of the proprietary hardware used to make them possible (no other company has replicated them successfully).
There are third party brands of component cables for the Gamecube, but these don't pull down progressive scan. Sometimes it's just a separate composite video/audio cable they attach to the other component cables for XBOX/PS2 combos.
In other words, these really are the only way to pull down a 480P signal from Gamecube games, except for the usage of a Wii (where the cables are much more abundant and do not require the proprietary chip as it's built into the system).
