I don't think expensive players are worth it in Fifa 14 unless its Ibra or Ronaldo.
You obviously haven't came up against a decent player using Neymar he is unstoppable in the right hands, scores goals for fun, again it depends mostly upon the player controlling him but I would say he is worth his high price tag.
I don't rate a lot of the expensive CAM's as being worth it though over cheaper counterparts, Guarin, De Bruyne, Boateng, Gerrard and Fernandinho are brilliant examples of really effective cheap CAM's.
Jovetic has just been simply outstanding for me in terms of value for money, he can single handedly win a game with how effective he is in and around the box and he is cheap so it's hard to swap him out for anyone I think the only person that could replace him is Ronaldo or Ibra which I could never afford.
I would swap Etoo for Neymar in a heartbeat though.
I haven't really came up against ribery or robben but people must think they are worth it to pay so much for them, so it's hard to say whether they are worth it or not compared to cheaper counterparts but if you were to look at what Menez for example can do for 750 coins, then for 750,000 coins I would expect the player to do a lot of the work automatically.
Kaboul is another example of this, he is so cheap and comparing his price tag to others yes they are better but if you look at the difference in price then nobody is worth it in terms of value for money apart from the decent cheap players