Arenado is tough to evaluate due to the opt out. FG did a good analysis of the value of opt outs here:
https://blogs.fangraphs.com/whats-an-opt-out-worth/
When the article was written, the "actual value" Arenado's contract was $9.6M/WAR, which is a good deal higher than the ~$8M/WAR SD and Phil paid for Machado and Harper. This means the Rox got worse than FA value on their Arenado extension...it was a bad contract the moment it was signed because the Rox are morons.
Then Arenado went out and posted a huge 2019, increasing the likelihood he opts out after 2021 from 40% to...something higher. Let's call it 50/50 now? Since his 2019 salary was so much lower than the rest of his yearly salaries, the Rox just used up the most valuable season of the contract, and it's still roughly valued at $9.5M/WAR.
In short, at $9.5M/WAR, the Arenado contract isn't as valuable as most folks think it is. In fact, it's not that valuable at all. Any team dealing for Arenado is doing so praying that he opts out after 2021, because if he doesn't that means the contract has turned into a disaster.