OK! magazine -- which reportedly paid Jessica Simpson up to $1 million to pose for half a dozen covers, not to mention a nice commission for her PR rep -- apparently isn't moving the merchandise. I hear that sales of the Jessica cover of OK!'s July 24 issue -- about 540,000 -- were 10% lower than the magazine's Jennifer Aniston-Vince Vaughn cover the week before, and 15% lower than the Britney Spears cover three weeks earlier. "Jessica was their lowest sale in seven weeks," a glossy-magazine insider told Lowdown yesterday. "She's so over. She's a bargain-basement star who can only sell $1.99 magazines at best these days."