That's a tough call, in part because Westbrook just hasn't found the end zone. Because I personally value starting RB's more than WR's, and because Westbrook has a lot of job security, I'd equate him with someone like Chad Johnson (who's having his own troubles finding the end zone). Maybe also Hines Ward and Joe Horn.
Others might disagree.
I don't think he's worth as much as the next tier of guys (i.e., Harrison, Moss, Owens, or Holt).
This valuation might be different if he were punching it into the end zone.