There’s really no way to know, exactly. The worth of an instrument is difficult to predict in such generalities. It would really depend on the make and the model. The value of a vintage Spanish guitar today wouldn’t necessarily be determined by it’s quality as much as it would be determined by whether or not it was sought by players or collectors. A quality guitar, that is well maintained and well cared for over the years, so that is as playable. perhaps more playable, than it was when it was new will have a certain value. Anything above that will be determined by how desirable an instrument, by players and collectors it is, I think.