ummmm, for the 55mm to stay at f/3.5 instead of f/5.6, it would have to increase MORE than it would for it to be f/5.6 (coming from f/3.5 at 18mm). If you stay at f/3.5 for 55mm, you need a 15.7mm aperture vs. a 9.8mm. Last time I checked, 15.7mm > 9.8mm. And more glass=more $$$
The increase in glass needed becomes greater when you are looking at longer telephoto lens.
Take a 70-200mm. If it is f/4-f/5.6, the aperture would have to go from 17.5mm (@70mm) to 35.7mm (@200mm). If you have a constant f/4 70-200mm, you need to have a 50mm aperture at 200mm. Now if you go to a constant f/2.8, you need a aperture that goes from 25mm to 71.4mm
I'm still not entirely sure where your confusion comes from. More glass=more $$$.