They didn't get £13m for Gyan, they made a massive loss on him. Looking at seasonal net spends is misleading for this purpose as it includes players brought in by previous managers and academy graduates (like Henderson who they got a massive fee for) and discounts what the players he brought in eventually left for.
Steve Bruce spent £70-£80m on players in his time at Sunderland, only 5 or 6 ever actually played regularly and of them all only the 3 mentioned prior actually sold for a profit.
They didn't make a massive loss on Gyan they got a £6m loan fee and another £6m when the deal was made permanent.
Though I missed an entire transfer window off the numbers I posted before, so it was bollocks anyway.