If you already have a cable connection why would you want to stream any live show through ROKU? The cable connection will provide a better picture and streams in real time, whereas streaming through ROKU introduces a time lag of around 1 minute.
A one-minute time lag may should trivial but it is not if you are trading stocks while watching CNBC. Being one-minute behind in responding to news can cost you big time!
If you want to stream live tv to save money you will need to subscribe to Sling, which is the only ROKU channel that streams the NBC shows live (with a one-minute time lag).