I think he's pretty decent value considering you can get him super late. You gotta consider, that he's possibly a #1 WR on a team that everyone is saying will be much improved. What's not to like? He's on a team with a solid, not great QB, a good running game, not much competition at WR other than an injury prone guy in Schilens and another unproven 2nd year player in Murphy. He's gonna get every opportunity to contribute (see D-Mac). I think as long as he continues to progress and stay healthy and with some stability at QB, he'll be just fine. Maybe not stud #1 WR, but servicable #1 to solid #2 WR.
I guess you could probably use Santana Moss's numbers over the years with Campbell as a starting point and equate from those numbers. Fairly similar offenses when you consider the Skins and Raiders both have stud TE's, good running games, improving defenses, and unimpressive WR's. Take into consideration that the Raiders are playing in one of the weakest divisions against some of the more porous defenses.
I think he's great value. You may have to sit on him for a few weaks. And once/if he has a few solid games, you could always trade him if you prefer.