Let's just say that whenever Spain didn't have the Lilis, Arantxas, Conchitas and the like there always was a legion of second-class players who didn't have the resources to travel a lot and, therefore, hardly ever competed abroad. Some of them, like Pilar Barril and Carmen Mandarino (50es to 60es) played RG and WB quite a few times. Barril, in fact, lost a three setter quaterfinal in 1959 or 1960. Those two players also won quite a few international titles even though most of them were of second tier if we compared them with today's ones.
In the 70es there were Carmen Perea and Victoria Baldovinos, who won one or two rounds here and there and the French Open, but they seldom played abroad. If I had to do a relative comparison I'd say that Pilar Barril would've been a top 20 player, Carmen Mandarino top 30-50 and Perea, Estalella and Baldovinos would've been top 50-100 players.
All in all I'd say there always was that second group of players. Even Lili Álvarez was joined by Bella Pons and some others at the Grand Slams (Bella Pons would've been in the class of the Pereas and the like). Those players didn't receive much in terms of money or resources to be able to travel and play. To be fair, Lili Álvarez came from a wealthy family and so did many of the players who achieved success abroad, whereas Carmen Perea was not exactly rich. In fact, I think the girls had to pay for their own trips to South Africa the year the Fed Cup was held there.
I'm not saying that many of those girls would've benefitted from some sort of scholarship. Maybe they were just not good enough to play at the highest level or maybe Spain has lost a lot of champions throughout the years.
I'm starting a compilation of Spanish female players based on the info I've got from this subforum and some other websites. I'll keep this thread active somehow