Re: "While the actual cost of supplying their services goes down"
Unless the contract specifically spilts out the costs as yours does and then keeps the internet charge constant, the lifetime price should be kept.
If the contract states its lifetime (for example 24 months) then their free to change prices after that point.
But if the contact doesn't expire, the cost is bundled into one cost and they promise 'lifetime' at said cost, I would expect it to stay the same.