James doesn't earn that much, Football Leaks released his contract and he earns around 3.8M/year NET or little less than 8M/year gross, so his salary shouldn't be a problem, but I think that he is impossible to get for us, he will go for 60M+ easily.
https://footballleaks2015.wordpress.com/2016/02/15/real-madrid-james-rodriguez/
Oh and to choose between Isco and James, that's very easy. James is by far better than Isco, he is the classic AM, has a deadly left foot and can assist for fun. Isco on the other hand is typical Real Madrid player, flashy, doesn't give a $#@! how team plays and is only concerned with how many players can he dribble, hogs the ball all the time, and is slowing down the play.