Flourescent bulbs are something like three times more efficient at converting electricity to visible light and last ten times longer. So if they're less than ten times more expensive than incandescent bulbs, it's already a better deal, and how much better a deal depends on how much you pay for electricity. Sure incandescent bulbs help heat a room, but they're less efficient than dedicated electric heat (because of the placement issue, if nothing else), and aren't tied to a thermostat other than the "nights are both dark and cooler" inherent usage. More than anything else, since electric heat is one of the least efficient thus most expensive forms of home heating energy, your incandescent light bulbs are less efficient heaters than whatever primary means you use to heat your home, it's just a question of how much less efficient.
