Time was, if you bought a used 360 hard drive, you could call a toll-free number and dupe Microsoft into sending a free transfer cable. Then they got wise and charged $12. It's now a fully listed accessory, for $15.
The cable went up on the Microsoft Store, sold separately, for $14.99 this past week. (You still get one free if you buy the 120GB drive from Microsoft). That's cheaper than the used ones you get from independent resellers on Amazon. (And the super cheapy kit does not work; it's for transferring gamesaves from a 360 to a PC.) The cable also appears to be a different configuration, if aesthetics matter to you for this.
When I upgraded to a 120GB drive earlier this year I bought it from an Amazon reseller whose listing said it included a cable, but it in fact did not. Rather than order one, which would have taken another 4 days to get it in my hands, I just put as much stuff as I could on a 512MB memory unit and re-downloaded the rest. I still lost a Rock Band 2 gamesave (and thus, all the standard songs I'd unlocked) and my Grand Theft Auto: San Andreas 100 percent gamesave, which really pissed me off.
Not that these things were unavailable back then, but for power users I can see this cable coming in handy down the line. And $15 isn't the kind of ridiculous price one might expect. It's still more expensive than free (or $12) but now there's no cloak-and-dagger or shady resellers involved with a product that should have been fully available on its own in the first place.
Microsoft Now Selling Xbox 360 Hard Drive Migration Kit [Blast Magazine]