It's a plausible question, but the answer will be a large number. Of course, one might argue that a billion or several billion flourescents is a much larger number than we were talking about. In that case we're talking about 1/2 a ton per billion flourescents, or maybe a tractorload volume of mercury.
Maybe the right conclusion is that recycling is a good idea since there are so many, but panicking over the 5 mg in a single busted bulb in the house is idiotic.
Especially since the household thermometer that we used to stick directly in our kids’ mouths actually has about 100 times as much mercury.
Even if every single lightbulb in the US is replaced with a CFL, and every single one of them is thrown in the regular trash, we will still have less mercury released into the environment than if we stuck to regular incandescents. (Power plants generating electricy for old bulbs create much more mercury pollution than could be released through CFLs in landfills.)
Environutalists get way too focused on the small stuff and never look at the big picture.