It's not a simple cable conversion. The N64 controller uses very different guts from Xbox and PS2 controllers. Just as an example, the analog joy on the N64 uses optical encoder wheels, like a mouse. Normal gamepads use potentiometers, like a PC or arcade analog joy. N64 converters have to use a real circuit board that does real work, not just cross-wire to a new plug.
As for the ebay ones, they are generally considered flaky, especially on the analog stick, rumblepack, and game save features. The Adaptoids have a reputation for working perfectly, just the way you'd expect them to.
I wish I could blow $144 on 4 of them. Probably not gonna happen, though.. Santa Claus ate all my money.