The youth system is a bit of a gamble, I haven't spent a penny on it yet - I decided to buy some trainees instead, they were expensive but the risk was much less which was the deciding factor for me - I didn't want to be left with no money and no decent trainees.
I've pretty much always invested the full whack in the youth scheme, probably since it's too easy to make the excess money each week. Over a coulpe of years, I would imagine I'm quite a lot up on my investment, but nothing huge - you seem to get a couple of excellent pulls each season who sell for £500k+ which make it worthwhile, but most of the time I get players that would sell for £10-100k. If you spend 12 ITS on a single stat, the worst they can be is mediocre - the best is respectable (1 net from proficient) - I believe that's in the rules, so if you pull a proficient (Newportlad), train train train and he could even make the U19 squad in a couple of seasons... I was able to use the full 35 ITS each week for the past couple of seasons since I built up a huge pot of around 200 (since I didn't have spare net spots), and I think they only recently introduced a cap (but im not sure it even works, since I still had 150+ at the beginning of this season).
If you can afford the 20k investment, I personally reckon it's well worth it - so long as you spread the ITS well, you very regularly get someone who sells for more than your 20k investment... but i guess it also depends on how much you've neglected your training facilities, and how much you'd have to pay to get it back upto superb. For some people, it just wouldn't be viable to spend countless numbers of 30k for this...