But, yes, in general, if you want to prevent it from being copied you would have to spend ever larger sums of money to accomplish that.
The entire point of copyright is a deal to encourage author's creating a work *so that it can enter the public domain*. This scheme is designed allow _most_ works to enter the public domain much earlier than they do now, while still allowing the most economically valuable creations to be pushed off for quite a while.
But, you the author wanting to prevent your book from being copied is a _harm_ to society. So, the goal with this scheme is that the extension costs should _roughly mirror_ those harms of continuing to lock a work up. The idea, then, is that when the economic benefit to you the author is _larger_ than those societal harms, then you'll pay for the extension.
But when those economic benefits to you the author is _less_ than those societal harms, then you'll stop paying for the extension.