You are correct that for 300M 10 year contract Cano would have to produce an average WAR of 6.
Cano could realistically put up a WAR of 7-8 over the next 3 seasons ... should he have the drive and dedication to the game.
We also have to consider that a unit of WAR will increase in value each year going forward.
What if a unit of WAR increases by 10% per year over the next 10 years. I think it is safe to assume that player salaries will continue to grow at a 10% rate. Of course we can argue that assumption as well. So lets break it down by year. I am just playing devils advocate here. I would never give this contract to Cano.
Year Season WAR WARV Value CUM Value
1 2014 8 5M 40M 40M
2 2015 7.5 5.5M 41.25M 81.25M
3 2016 7.0 6.06M 42.35M 123.60M
4 2017 6.5 6.67M 43.33M 166.99M
5 2018 6.0 7.34M 44.02M 211.01M
6 2019 5.5 8.07M 44.41M 255.42M
7 2020 5.0 8.88M 44.39M 299.81M
8 2021 4.5 9.77M 43.96M 343.77M
9 2022 4.0 10.72M 42,87M 386.64M
10 2023 3.5 11.79M 41.27M 427.91M
You see Hal Steinbrener we are actually giving you a home town discount of about 130M. almost 1/3rd off Cano's true value.