OK lets put Stones in the standard young purchase category, I still disagree as he did have some not insubstantial first team action with Barnsley at Championship level unlike the likes of Green, but anyway the hit rate for these players is still extremely low. You already mentioned Green who has not really kicked on at all, I could also throw in Duffy, the previously mentioned Mustafi and Dier, even Gueye and Vellios who may now seem like jokes, but at the time of signing were viewed as players with potential.
Now contrast that low rate of success with the experienced established internationals players we have signed. McCarthy, Mirallas, Oviedo (but for injury) and going back further Fellaini, Distin, Jags and Baines. A couple of the loans have failed like Drenthe and Jo but any purchased player with a decent pedigree (i.e. substantial playing time and/or having played for their country) have in the main been very successful for us.