Well net spend works more or less in same way, club sells bunch of nobodies (who are never part of first team) for desperate club for a decent fee and then somehow turn that into "We didn't spend much, because net spend"
Big clubs rarely lose their best players against their will, like Ronaldo with us, Hazard with you people. Most of the times net spend argument is based on who can sell their squad players and fringe players for good money.
I agree with
@WeePat , just take the total transfer cost to assemble the squad on first day of the season and that's the money spent. How you made the money shouldn't matter. End of the day if you have 500 million worth of players, same as Manutd then it shouldn't matter whether you made 500 million by selling players or noodles. This isn't exactly reliable method too as transfer market changes so much every summer, so club that started the rebuild latter end up playing more money.