I think @mibike has it right with his estimate, since I've seen some charts showing the typical drag of 2-3 watts (hub dynamos). To be honest, to me it does seem like a lot to give up when you're looking at .25 mph difference. It takes some work to raise your threshold power enough to increase your speed that much. In theory, thinking about it. But then I get some perspective when I realize that I haven't aired my tires up in a week, or two weeks currently, and that costs me quite a bit more than that and I don't seem to care in practice. Or buying a faster tire will gain the 6 watts, and that's never happened either. I could probably find a couple of watts in my drive train. Even though I think I care, in practice it's obvious I don't. If you're at all like that then I think it looms larger in our minds than it does in actual practice.