I think the Padres are getting a solid deal. The reason why is they aren't eating bad years that players usually get tacked on the end of the contracts. Tatis's contract only goes until he is 35. Most of the players with big deals are paid until 38yo+ which contains years where the players are sometimes worse than a AAA player. Ex. Pujols, Cabrera, Arod
If he stays as the same player he is right now he is worth his deal and he should project to only get better over the next 6-8 years. He won't stick at SS during the whole deal and that hurts his value a bit but he can easily slide over to 3rd at the end of Machado's contract or DH.
I know Tatis isn't a FA or even close to one like the other contracts I listed below, but it will be interesting to see how they bought out his Arb years and what they are paying for his FA years. I would think he's getting around 40m for his Arb years, then 300m over the last 10 years,
Trout: 12yr/426m (38yo)
Betts: 12yr/365m (39yo)
Harper: 13yr/330m (38yo)
Stanton: 13yr/325m (38yo)
Cabrera 8yr/248m (40yo)
Pujols 10yr/240m (41yo)
those contracts were signed with the team knowing that those players aren't going to worth what they are being paid at least the last couple years and potentially longer. Tatis's deal doesn't have those bad years for the team that they normally have to eat to secure a long contract.
I'd assume Juan Soto is going to aim for a similar mega deal.