I think it's hard to say how good a fee it is unless you've watched him quite a lot. If he's as good as some suggest then it's a bargain. If he's as bad as others say, it's a disaster.
In principle, though, I think £17m or so for a top young full back is perfectly ok.
Oh, I agree certainly with the idea of paying 16-20 million for a top prospect who can solve a problem position for the next 6-8 years.
I'm just not convinced he's a top prospect. I tend to keep my eyes and ears open for prospects around the continent, and I'd never had heard of Moreno before he was linked to us. There were then certain stories about him joining Real, but agent-generated hype can be a great thing. A case in point is someone like M'vila - who, 3 years ago, was touted as the best CM in Europe - certain to join Real for 25 million. Today, he can't wait for a team to rescue him from Rubin fucking Kazan. Of course, that may be a case of attitude rather than talent issues, but I think the point about hype still stands.
With Moreno, I've not really read anyone say great things about his qualities - someone posted the Sevilla fans' reaction to him leaving - there were no less than TEN fans - and not one of them thought he was great. Every single person on here who saw him play in the Europa League final thought he was shit. And then there's Youtube - several videos of him have surfaced over the last couple of weeks - and having pored over 5-6 of them, I can't identify any particularly outstanding attribute.
I think when you're evaluating a 21 year old prospect, you really need to look more for attributes - ability on the ball, pace, and strength - rather than game intelligence or consistency (which come through coaching and experience).
With this kid, all I can see is that he has a nice touch and is comfortable on the ball (which I'd expect from most Spanish based players) - he doesn't seem to have great pace, great strength (in fact he seems to have some puppy fat) or very good centre of gravity (which I think is critical for full-backs who need to bomb up and down the wing and change angles constantly).
Perhaps he's in the Neville/Finnan mould of a very consistent full-back who makes no mistakes - but of course we'd need to see him over the course of a period of time to make that assessment. I just feel that based on the little information we have, this seems like a big gamble.
There's a lot talked about in this forum about trusting the management - but given what our Spanish based scouts came up with last season, trust is the last thing I have in them!