The same amount as xhakaGo look up how much United spent for Bailey and then let me know if you would classify that as finding a diamond in the rough.
And I was a big fan of Zouma before his injury. Man has the potential to be a beast. Hopefully he recovers quickly and reaches his potential.
As far as the goals conceded - that goes back to my point about your managers playing style. Of course you will concede less goals if you park the bus every week. That doesn't mean you have a good defense. But I am sure you know analyzing something like defense goes well beyond one stat.