How many points would the rooks be worth in this starting position?

Would just you give the rooks a value of just 0 points based on being immobile, or would you give them a negative value based on hindering the movement of the black pieces more than the white pieces?

They would be worth 30 points

The rooks have a negative value. Black can only play around them while white can drill through whenever desired. Effectively, white has more space and more options.

I am sure you can create positions where white wins easily after you expand the rooks territory to somewhere near 15 rooks.

Probably 10

Not much, they are all immobile and the Rs at e5 and e3 are sitting ducks if White decides to attack them.

I am not talking about attacking the rooks but about eliminating the black king. 

I think this sums it up

Yeah me to

Yeah and they can only move three ways