The easiest answer, I'd say, would be to have it give a similar but weaker benefit to all of your cities (weaker, because if you then build the Broadcast Towers yourself, you don't want it to get too out of control). For instance, just a flat "+1 happy per 20% culture" in all cities, or "+1 happy per Hit Movies, Hit Musicals, or Hit Singles".
Now, personally I don't have a problem with the free-building Wonder concept. Wonders that give you a straight bonus give you power, while these Wonders only give you time. After all, the other players could just build the buildings themselves. For large, core cities that have plenty of production, it wouldn't take that long to build Broadcast towers anyway, so really what you're doing is sacrificing a dozen or so turns in one city to save three or four turns each in a larger number of cities; it's still a net gain, of course, but once the other civs build the towers of their own, your only real advantage was what you built in the turns you would have used for towers of your own. And then there's the risk factor; will you choose not to build Broadcast Towers while you build the Eiffel, knowing that if someone beats you to it you'll be further behind, or will you build the towers anyway, knowing that you've wasted effort if you do get the Wonder?
It is kind of strange that they removed every instance of this type of Wonders when translating old wonders to Civ4, but then added a new one. But the bigger headache for me is that the Eiffel Tower is unlocked one tech BEFORE the building it creates, which is the complete opposite of the Civ3 pattern. Granted, you'll probably want to research Mass Media anyway (for the U.N. and Hollywood), but I'd want to change its effect to avoid this problem.
