so you're looking for a real video card, not an addon card that allows you to watch TV etc, like you asked, but only on the main monitor?
all I can say is good luck finding a solution for under $100 (US). I think you'd be best off to buy an AIW card (I don't know if they have PCI AIW cards) as the second video card, but even then, I cannot and would not trust it to work properly in a dual monitor setup. BOTH ATi and nVidia don't appear to test their cards (or their drivers) in dual monitor configurations, so half the time, it's pure luck getting all the features of both cards working.
the best dual monitor solution though that I could see, would be using an ATi Radeon 8500DV (it has dual monitor support, and all the AIW stuff AFAIK). another card you might consider which has dual monitor support, though no AIW stuff is the Radeon VE, which is a Radeon with crippled 3D, but also the first Radeon to support HydraVision (their method of dual monitors on a card). add in the TV wonder, and you probably would get similar results as the full blown Radeon 8500DV, only with alot less 3D performance..