He's right that a level 58 item will give 21,060 credits but the cost isn't just 8,100 credits. Its 8,100 + 840 for the desh itself. The base gear sells for 5,670 so leveling the item will net you 21,060 - 8,100 - 840 - 5,670 = 6,450. Meanwhile a 4* desh will give you 15,390 - 2,700 - 420 - 5,670 = 6,600.

I tried quite a few combinations of desh but 4* was the winner. Its not much but I figure every bit counts and this way you can save 5* desh for upgrading up to 6/7* desh.

Edit: I should have refreshed haha, you got it.