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).