You cannot just quote player fees, add and subtract and get to a net figure
you have to factor in wages, agents, image rights and above all, amortisation over contract length
it. Makes. No. Sense. To. Quote. Net. Fee. Spend.
It does make some sense to look at it, as it gives a cursory guide to how much a club is spending. Generally, if a club has a high net spend and they have a high wage bill, then they are spending a lot of money, right?
In terms of affordability, then it doesn't make sense to say "oh, they spent £100m net, they must have to sell" because, as you say, there are other factors.