I'm not sure what metrics you're using for "spent around the same amount," but from preliminary looks at AC Milan vs. Brighton that doesn't seem true at all.
Breaking things down into transfer vs wages: over the past two seasons, Brighton have a net transfer profit of ~80m euros per Transfermarkt, while AC Milan have spent ~110m net, so 190m difference. (Stretching back further, you get to some seasons of 60m net spend by Brighton while they were trying to get promoted, but this is during a time when AC Milan's ownership spent 100m+ net a few seasons.)
And on wages, Milan pay substantively higher: Swiss Ramble, probably the top public football financial analyst, has Brighton's wage bill at 109m pounds (https://twitter.com/swissramble/status/1512310039956049920) for the 2021-22 season and AC Milan's wage bill at 169m euros (https://swissramble.substack.com/p/milan-202122-finances).
Now -- Brighton are shrewdly run, but I doubt it's sustainable for any club to consistently show the kind of short-term player trading profit that Cucurella + whatever they get for Caicedo and Mac Allister represent. Given the owner's background, I would expect Brighton to continue to be solidly above-average, but other clubs have data analysts and scouts and such too (and are capable of just copying Brighton's strategy inasmuch as it's reverse-engineerable from outcomes).
Breaking things down into transfer vs wages: over the past two seasons, Brighton have a net transfer profit of ~80m euros per Transfermarkt, while AC Milan have spent ~110m net, so 190m difference. (Stretching back further, you get to some seasons of 60m net spend by Brighton while they were trying to get promoted, but this is during a time when AC Milan's ownership spent 100m+ net a few seasons.)
And on wages, Milan pay substantively higher: Swiss Ramble, probably the top public football financial analyst, has Brighton's wage bill at 109m pounds (https://twitter.com/swissramble/status/1512310039956049920) for the 2021-22 season and AC Milan's wage bill at 169m euros (https://swissramble.substack.com/p/milan-202122-finances).
Now -- Brighton are shrewdly run, but I doubt it's sustainable for any club to consistently show the kind of short-term player trading profit that Cucurella + whatever they get for Caicedo and Mac Allister represent. Given the owner's background, I would expect Brighton to continue to be solidly above-average, but other clubs have data analysts and scouts and such too (and are capable of just copying Brighton's strategy inasmuch as it's reverse-engineerable from outcomes).