I don’t really get this net spend argument when determining if they have spent enough money on their team. What players have Chelsea sold that they wanted to keep? Selling Oscar for £50m doesn’t mean they didn’t spend it to attempt to strengthen their team.
It's to do with status quo. If you've got a high net spend it means you're investing a lot of money into bringing in new players while also selling off low valued players. In theory, that should mean you're improving your squad as you're replacing low valued players with higher valued players (doesn't always work out that way, see AC Milan). If you've got a positive net spend then you've been weakening your squad as you've not been investing enough back.
It paints a better picture than just spend as it takes into consideration both players coming in, but also players going out of the club.
Players In (value over 10m Euros):
Álvaro Morata - 66,00 Mill. €
Tiemoué Bakayoko - 40,00 Mill. €
Michy Batshuayi - 39,00 Mill. €
Diego Costa - 38,00 Mill. €
Danny Drinkwater - 37,90 Mill. €
N'Golo Kanté - 35,80 Mill. €
Antonio Rüdiger - 35,00 Mill. €
David Luiz - 35,00 Mill. €
Cesc Fàbregas - 33,00 Mill. €
Juan Cuadrado - 31,00 Mill. €
Pedro - 27,00 Mill. €
Davide Zappacosta - 25,00 Mill. €
[Irrelevant point] - 23,00 Mill. €
Emerson - 20,00 Mill. €
Abdul Rahman Baba - 20,00 Mill. €
Filipe Luís - 20,00 Mill. €
Olivier Giroud - 17,00 Mill. €
Ross Barkley - 16,90 Mill. €
Loïc Rémy - 13,20 Mill. €
Asmir Begovic - 11,00 Mill. €
Players Out (value over 10m Euros):
Diego Costa - 66,00 Mill. €
Oscar - 60,00 Mill. €
Nemanja Matic - 44,70 Mill. €
David Luiz - 49,50 Mill. €
Romelu Lukaku - 35,36 Mill. €
André Schürrle - 32,00 Mill. €
Ramires - 28,00 Mill. €
Nathan Aké - 22,80 Mill. €
Juan Cuadrado - 20,00 Mill. €
Filipe Luís - 16,00 Mill. €
Mohamed Salah - 15,00 Mill. €
Petr Cech - 14,00 Mill. €
Ryan Bertrand - 13,34 Mill. €
Asmir Begovic - 11,50 Mill. €
Bertrand Traoré - 10,00 Mill. €