This article won't be too in depth, but it will generally give a good summary of this deal and whether or not it is a good deal.
Fernando Tatis Jr. just signed a 14 year, $340 million dollar extension with the Padres. At first glance, this is a comical steal. From ages 23-36, a healthy Tatis would be expected to post around 84 Wins Above Replacement. That is, 6.5 WAR per season until age 30 when he loses half a win every year from that point onward. Using this calculation, the Padres will be paying around $4.03 million dollars per WAR produced by Tatis. The current pandemic-impacted market is around $6.4 million/WAR. On the surface, this looks like a huge steal for the Padres. Does this hold up?
For one, it is a little unfair to assume that Tatis will play 150 games per season for the rest of his career, as is expected in the original calculation. I don't have the data at hand, but I doubt that many players played 150 games per season from ages 23-36. Now that I think of it, I actually can check this. Give me just a second... I got it. 150 games per season for 14 years equates to exactly 2100 games played. Since integration, just 14 players have surpassed 2100 games from ages 23-36. All of them besides Pete Rose and Rafael Palmeiro, who were hall of fame talents, seem to be Hall of Famers. The most recent stretch was Albert Pujols playing 2108 games from 2003-2016. Tatis will be good enough to get that many starts, that is not a concern. The concern is that he would be healthy, especially later in his career. To get an arbitrary but reasonable number for his games played, I will look at Alex Rodriguez' games played from ages 23-36. He played 2011 games, or 143 per season. Tatis and ARod are both very similar players, so I think this works out.
Using said playing time total, Tatis is now "only" projected for 79.5 WAR, which comes out to 4.27 $/WAR. This still remains a massively team-friendly contract, so what could we be missing? Well, the Padres already had 4 years of control over Tatis remaining. He was still going to make money, but a lot less than what he would make on the open market. The next step of this examination will be to calculate how much he would have made over the next 4 years.
It isn't entirely easy to do so, but a good comparison would be Mookie Betts of the LA Dodgers. If not for the pandemic, Mookie would have made around $58 million in his last 4 years of his rookie deal. Only 3 of those years qualified for arbitration, and I'm pretty sure that Tatis qualified for the thing where a young star gets 4 years of arb and just 2 years of getting paid nothing. If this is confusing, I apologize. Anyways, you take the 58 million that Mookie made and then take Juan Soto's 2021 contract, as he is on the same track as Tatis, and you get 66 million. Inflation is a thing, and Tatis probably would have made a little more money than Mookie as a result, so let's just say Tatis was essentially on a low-risk, 4 year $70 million dollar deal. In practice, the Padres replaced that deal with a 4 year, $100 deal in exchange for a 10 year, $240 million dollar extension. To put it simply, they basically just gave him a 10 year, $270 million dollar extension on top of what he would have gotten already. If the Padres signed theoretical 26 year old Fernando Tatis Jr. to a 10 year, $270 million dollar extension, and he played 143 games per season, they would be expected to pay $4.9/WAR. I think this is the best measurement for the deal and it remains a huge steal.
Tatis provides value beyond his WAR as well. For one, concentrating 6.5 WAR into just 143 starts is more valuable than signing 3 separate 2.5 WAR players and getting 7.5 WAR in around 430 starts. Playing time, just like WAR, is a scarce commodity, and a true superstar player gives a team the chance to give playing time to different positive WAR players and ultimately win more games. This is difficult to articulate without using poor and incorrect logic, but basically a team that has the ability to give more playing time to positive WAR players benefits from concentrating resources into one 6 WAR player instead of spreading it across multiple 2 WAR players. For a team without the depth necessary, it does not matter. However, those teams won't be competing for playoff spots anyways.
Tatis is arguably the face of baseball and easily one of the most marketable athletes in the game. The value he provides when the Padres gain popularity as a result of his name and performance is completely unknown, but there is definitely additional financial value in having a guy like Tatis on your roster.
This article got off the rails, but the point is that the Padres did a great job with this Tatis extension and that they set themselves up handsomely for future success. As for Tatis, he did just fine. $340 million dollars is a lot, even after like half of it gets removed for taxes. Considering the diminishing marginal utility of money, I doubt that the difference between $340 million and around $600 million (which is probably what his market value is, lol), is a big deal. He eliminates all of his risk and gets to play baseball in San Diego for 14 years, which sounds fun enough.