tbf, a lot of davek's stuff is utter nonsense but that logic in regards to spending is correct. Net spend could be seen as a measure of Moshiri (for the record, I think he is doing fantastic stuff for the club), Purchases is a measure of what Koeman and Walsh should be judged on.
If guys like Niasse aren't good enough, if Delefeou isn't good enough, if allowing Lukaku to leave, if scaring Barkley to leave, if Lennon is allowed to leave and not be in the first team then damn right Koeman should be judged on his replacements.
Last year he sold Stones who has proven to be a potato and was a liability here, he brought in Bolasie who was quality til' he got injured and he brought in Williams who has been rubbish. He then brought in Lookman (who has made good progress and has supporters buzzing) and Schneiderlin who is one of our most important players already.
What Koeman cannot do is go down the path of AVB and waste what money he has been given (i.e the Bale money), which is why he should be assessed on what he buys with whatever he is given.