Hi Martin,

I looked into this a couple of months back as a friend had an old Estudio Contreras, and also Gear Junkie was selling a very similar model at about the same time. It depends a lot on the model that you have. Here is a discussion thread which might help shed some light for you:
[https://www.classicalguitardelcamp.com/viewtopic.php?t=138628]

Essentially, if you have one that is signed by a particular maker, this indicates that it was hand made, and these are consequently much more valuable. The Estudio models are factory made and the value is dependent on the condition just like any other mid range classical guitar. Soundboard bulging behind the bridge can make older classical guitars difficult to play.

I hope this helps! Please post here if you need more help or find out anything interesting about yours.