According to The Vertical's Shams Charania, the deal is worth $72.3 million over three years.
Under the terms of his previous contract, Aldridge had a $22.3 million player option for the 2018-19 season. According to ESPN's Adrian Wojnarowski, the extension included that option:
Adrian Wojnarowski @wojespn
Aldridge extension starts in 2019-'20. He will opt into his $22.3M for 2018-'19 season and will be signed through the 2020-'21 season.2017-10-16 20:33:57
Wojnarowski also reported San Antonio had included Aldridge in trade discussions with other teams but was unable to find a deal it liked.
"San Antonio doesn't have a better alternative available to them in free agency to partner with Leonard," Wojnarowski wrote, "and Aldridge couldn't be sure that he'd be able to find this kind of long-term financial score available on the free agent market."
While this extension resolves Aldridge's short-term future with the Spurs, it's somewhat surprising. The five-time All-Star has had an underwhelming two-season tenure in San Antonio.
Aldridge has averaged 17.6 points and 7.9 rebounds per game for the Spurs. In nine years with the Portland Trail Blazers, he averaged 19.4 points and 8.4 rebounds.
His unsuitability as a No. 2 option behind Kawhi Leonard was abundantly clear in last year's playoffs as well.
Leonard's ankle injury provided an opportunity for Aldridge to put the team on his back. Instead, San Antonio had a minus-4.8 net rating when he was on the court in the postseason, according to NBA.com. The Golden State Warriors swept the Spurs in the Western Conference Finals.
The extension also limits San Antonio's financial flexibility for the next three years. The Spurs have $38.3 million tied to Aldridge and Pau Gasol in 2018-19 and then even more committed to the pair again in 2019-20 depending on the specifics of Aldridge's contract.
With that said, the Spurs may have had trouble finding a replacement for Aldridge at roughly $24 million a year. The Aldridge of the last two years is still a skilled—if somewhat flawed—big man. If Paul Millsap can get $60 million guaranteed for two years—with a $30 million team option in the third year—then $72.3 million over three years for Aldridge is more than defensible.