Just checked on line auctions, past 90 days for S&W Model 66's that have sold- depending on condition, around $400-$450.
Value is more determined on condition. The length of barrell and serial number are helpful but the Model of the revolver is essential. Without knowing the model number the question is almost impossible to answer honestly.
I will assume that this firearm was made by the Spanish firm AYA as they did make a model called the "Matador" from the mid '50's thru the early 1980's. If this is the case it should also, I believe, have either AYA or Aguirre Y Aranzabal stamped on the firearm. The Matador was somewhat of a "knockabout", though quite serviceable, grade of shotgun for AYA. The "Blue Book of Gun Values" gives values of from $200 to $795 depending on the particular grade and condition. I believe these figures may be OK for the low end, my feelings are that a 100% gun would sell for a couple of hundred bucks more at most local gun shops. They were good guns, I almost bought one myself 25 years ago and if my memory is correct the price them was just under $500 for a used, but still quite nice gun.