Every contract your interact with will cost gas (which costs ether), and I am unaware of any scheme under which these costs would not rise linearly with the number of subscribers.
Each time you execute another contract you will be looking at a baseline of 21,000 gas plus whatever gas the actual execution requires. In perfectly optimal conditions, your upper limit for number of contracts that you could call in a single transaction is going to be
blockGasLimit / 21000 which currently works out to approximately
I would assume any such service applied generically will likely need to ensure that gas costs are paid by the subscribers. In this case, you probably want something akin to this proof of concept contract.
This gas proxy is guaranteed to repay the caller all gas costs. With some access control in place to ensure that the contract is only called in appropriate conditions this type of contract could be used to keep the costs for gas on the subscriber's side.