It is impossible to quantify. You cannot assume that a particular error in a game 'cost us 2 points', because if that particular error hadn't occurred, then the entire rest of that game is different. The opposition may well have still won/drawn the game, making the 'error' redundant.
All we can really say, is that Vito Mannone is definitely a better GK than any of the 3 on our books now, which is ridiculous when we only got 2m for him and spent half of that on Steele (who is now 3rd choice) and Vaughan (who was in and out quicker than hooker's punter).
In replacing Mannone, we've spent more than 25% of the fee received, and are paying a wage to 3 goalkeepers, all of whom are worse than him.