15 million is on the high side for Joe, but there weren't any other options given that BR wants someone who knows his system and Swansea didn't need to sell. The crazy transfer fees of late haven't helped either, not to mention the terrible precedent that Liverpool has set when it comes to signing players (especially last season). In fact, I think the players we have moved on for a profit in the last decade can be counted with fingers and toes. 

However, I do believe a large part of Joe's signing on fee can be justified by his suitability and knowledge of BR's system. In the current football clime, where the clubs bankrolled by oil money are signing all the biggest names, it is nigh on impossible to compete with them in terms of individual talent. Therefore, the only way to compete with them is to ensure your system allows the team to be more than the sum of its parts. In this respect, getting a player who is suited and is already used to BR's system might well be worth the extra money. A negative example of this would be Adam who played well at Blackpool because their game was played to his strengths (not many I'll admit); We all saw how badly he floundered at Liverpool where he was placed into a different system. 
Currently at center mid we have Gerrard, Shelvey, Lucas, Henderson and Spearing fighting for 3 spots. Of these five, only Gerrard and Lucas deserve to start in the first eleven. Shelvey shows potential but is still too raw. Henderson has more stamina and is a better tackler than Shelvey but he really needs to work on his positioning and movement. Spearing should be shipped out. A willing and hard worker, but hardly a reliable back up. Even with Joe's arrival, we are still looking light in midfield if any of our starters pick up a long term injury. 
This holds true for our center backs as well. Carragher has been on the decline and can no longer be seen as a reliable back up so if either Skrtel or Agger pick up a long term injury then we are in trouble. Coates, same as with Shelvey, is too raw to fill in as of now. 
As for our wings, we should look out for another player in Borini's mould, who can play interchangeably among the front 3 as a winger/forward. It would be a bonus if he could also function in an AM position, which would allow our front 4 to play interchangeably as well as provide backup for Gerrard, who certainly can't play every game. With such a system in place, it is obvious Downing has no business here because his game is built upon driving towards the byline, not to mention his glaring lack of intelligence when it comes to making the final pass. Dempsey is a possible solution, not the best because of his age, but he has excellent football intelligence (positional awareness, movement, vision) and also offers an aerial threat.