Tom Cruise's Left Nut

Why is it that people (guys, usually) would "give their left nut for [something good]". Why not their right one? How is it that value is assigned to each nut separately? Furthermore, how can one nut be deemed so worthless in comparison to the other that they'd give it away in exchange for something else?

Now, I would hope that what they would give it away for is actually quite important, but you never know. On the weekend I heard Tom Cruise say he'd give his left nut to become a cadet major ("Taps", 1981). Maybe his nuts aren't worth all that much to him or something, but it doesn't sound like a fair trade to me. I'm sure there are some guys out there with the power to make him a cadet major that might call him up and say, "So, were you serious about that?" Now what they'd do with Tom's ball is beyond me... Maybe frame it? They'd have to flatten it first.

Then take the Chinese eunuchs as another example. They gave up BOTH their nuts just to serve the Emperor of China. They must have wanted that a little more than Tom Cruise wanted the "perks" that went along with moving up in the marine corps. Still... if I was browsing monster.com and I happened to come across a posting that required some light travel, experience with MS Office and my left nut, I'd probably pass.

I, for one, wouldn't give my left nut for almost anything. I say almost because maybe I'd give it away in exchange for a) the same nut right back in the same place and otherwise intact plus b) a cool million dollars. But actually, in that event I'd have to hope that whosoever's offer I accepted actually didn't try to collect the nut. I'd hope that they'd say, "Here, just take the million and we'll call it even. But I want the attaché case back; it wasn't part of the deal."