Contract is much cheaper after 1st year because signing bonus is already paid by prior team (similar to Josh Rosen).
This is why trading him after the 1st year is the worst possible thing for the Giants/Gettleman to do.
They paid all the up front money but then lost all value for the rest of the contact.
Here is the breakdown:
https://www.spotrac.com/nfl/cleveland-browns/odell-beckham-jr-14421/
"Odell Beckham, Jr. signed a 5 year, $90,000,000 contract with the New York Giants, including a $20,000,000 signing bonus, $65,000,000 guaranteed, and an average annual salary of $18,000,000. In 2019, Beckham, Jr. will earn a base salary of $16,750,000 and a workout bonus of $250,000, while carrying a cap hit of $17,000,000 and a dead cap value of $19,500,000"
2019: 17mil
2020: 14mil
2021:16mil
2022:15mil
2023: 15mil
Plus there is an out after 2020 with zero dead cap.