Out of the current Spain squad, 18 out of 23 players come from the academies of the richest clubs in the land, however (Barça, Madrid, Sevilla, Valencia, Atlético).
Only David Villa comes from a small team academy (Gijón).
It is true that the academies of these rich clubs have become really good at scouting and snapping up prospects in youth tournaments, however.
I agree on the overall point that it's cheaper for Valladolid to field a bunch of local lads than buying Arriaga, Arriaga II and Barriaga.
As I said I did no research into whether its actually correct or not, just a general thought i had. Arriaga II could definitely do a job for them though.