I keep getting the feeling that this is all nothing more than a test and a way to find out how deep Monica is willing to fall. Maybe it's revenge on her husband's part, maybe it's something else if we assume that the husband is acting in concert with the Farmers, but my conspiracy theory is that Monica is likely to meet her missing husband at the very end of the game, and the way she acts during this casting could have a definite impact on the outcome of that meeting and on one of the endings of the game. I.e. my point is that the idea of casting husbands might not be at all what most people think it is (Monica and one of those assholes getting married).
As for the money, your suggestion of a photo shoot/product promotion makes sense, but I'm leaning toward the other option - the money issue is probably just a way to buy Monica's loyalty to Campbell so that when X time comes, she'll choose him. Of course, we'll be shown Monica's sex scene with Campbell (and knowing his proclivities, we can assume that other people will be involved), and the money briefcase will serve as an incentive.
That is, the briefcase could serve two purposes - to win Monica's future loyalty by capitalizing on her need for money, and at the same time to induce her to do something particularly perverse during her date with Campbell. Monica no longer reports to Biff and is now technically back in charge of her magazine, and Campbell has repeatedly stated that he is a staunch opponent of blackmail and coercion, so instead of blackmail, "good old-fashioned bribery" with money will be used.