To explain it, let us take a look at this mockup…
In the middle, the crash indicates the time disaster struck. When we go back to the latest point we took our backup off-site (in regards to the affected crash site), where this point should be less than the maximum tolerable period in which data might be lost from an IT service due to a major incident. So the arrow towards the left indicates the RPO. Be aware that storing these backups on the same risk site does not fulfil your RPO!
The arrow to the left indicates the time needed to restore the service. Be aware that this is from a non-technical / business perspective. So merely starting up the system is not enough. Users of the service need to be able to use it again!
In terms of costs, be aware that strict objectives will imply more expensive solutions. The closer you want the objectives towards your crash zone, the more expensive it will become. An RPO of 7 days will still allow tape backups to be taken offsite once a week, where 1 day will still allow a nightly replication, yet where shorten time constraints implies near online replication.