I signed a contract with a roofing company to do work inside and outside of my snroom. He told a sub contractor to come over and see the damage that was done inside the home. Sub said he was only getting paid $300 when damage was over $1000, I then hired the general contractor for inside work. Is the contract I signed for the roof and inside work null and void now. He is wanting to charge me the contract price for work that I found out only costs $900 for the roof.

No, you signed a valid contract, if you agreed to a price that was too high, that is really not a issue, he is allowed to charge more if you agree.

And sub contractors always get less than contractor, if you hired another contractor to do work you already had a contract on?

Did you give them written notice to cancell the contract ? You can be liable for costs that the contractor had ( labor and material) if he acted on the contract.

And from state to state,you have a certain amount of days to change your mind and get out of any signed contract..

Probably too late for this information as in Illinois you have 3 days for this to occur.