A little more background.
Silica is indeed more expensive than carbon black, but not enough to worry about - and certainly not enough to be the difference between the price of tires.
Where the problem lies is in making some part of the carbon black load rubber in contact with the road surface without appreciably affecting the silica loaded rubber. This requires some specialized manufacturing equipment - and the difference in price is more or less a reflection of the increase in quality. And certainly, low end tires would be made in plants without this equipment.