Ah crap, I flubbed it - was looking at net salary for Werner and pre-tax for Luiz. Werner is on £14m per year pre-tax (including his signing bonus amortised).
Perhaps I explained this poorly - it's because when a player signs a multi-year deal all of the fees associated with the transfer are amortised over the length of the contract. Because Luiz signed a 1 year deal (with an option) initially, there's no amortisation and thus it's far more germane to include all the agent fees and whatnot when discussing his annual cost. Obviously now that Arsenal have taken up the option it looks a bit less egregious, but I just took issue with the notion that he's a bargain.