Here we go:
CNBC Link
It's from a bit over a year ago, but the guy writing the article covered the signing in the first place, and as such should have the skinny.
Oddly enough, LeBron's original contract ends this summer, completely obviating the (apparently nonexistent) major-market kicker anyway.
That's not to say that LeBron might not get a bigger Nike (or other compnay?) deal this summer if he goes to a major market. I personally wouldn't expect that to amount to much, though - everybody in the world knows who LeBron is. At some point there's got to be diminishing returns for media saturation. The ad companies would know this better than I, but I imagine LeBron is already saturated no matter what market he's in.