In my opinion the best value/money investment is 5* veteran player, if you are 35T (or some few more) player.
The problem is that you must refresh your team because those players last for 2, max 3 seasons.
So you must involve in many auctions.
The best investment if you have extra T, is scout/list player . The problem is that very few of the list are worthy (2-3 of 30 players)
Also you have a possibility (50/50 ?) that your player will perform bad.
The best investment if you have sources (greens & reds) is nordgens. The young expensive fast trainers.
The problem is that they are rare.
Academy players is the worst investment of all, but maybe try some time for fan.

If you want, check my latest post in this guide
http://forum.topeleven.com/tutorials...tml#post347704 #253