I realize I may be asking something for which there may be no simple answer.
From what I gather, there were some dark days in Spanish gun-making. What periods/manufacturers is it advisable to avoid? What are the best values in Spanish field grade and mid-grade guns?
You're right, there aren't any simple answers :-(
Let me provide some background for context.
Spanish shotgun makers are generally artisanal, meaning each gun is individually made to a customer's specification and for a specific purpose - and at a specific price point. The price point is largely determined by how much hand labor is going to be required to make the gun.
Let's do side lock guns.
Here is a low price point side lock:
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=416923154 What makes this a low price point gun is the engraving, which is mostly roll marked.
Here is a medium price point gun:
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=417168740 Hand engraved gun, but the engraving pattern is simple and coverage is only in the 70% range
This is a very high price point gun:
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=417947846 Engraving pattern is fine, complicated, and coverage is 100%.
Now: two of the definitions of value that I use; gun value and social value.
Gun value is determined by how well the gun works. Guns that work well, seldom need repair, and last forever have high gun value. All three of the above guns have equally high gun value, as field guns.
Social value has a slippery definition. In short, the more people who stop and go "Oh, wow!" when a gun is uncased, the higher the social value. Price point tends to follow social value.
Another factor that has an impact on gun price point (and re-sale gavel price) is the purpose to which a gun was made. Upland game hunters tend to pay a premium for light game guns (a light game gun in 12 gauge will be chambered for s 2.5 or 2.75 cartridge, and weigh six to six-and-a-half pounds). Competition shooters, especially live pigeon shooters, will pay a substantial premium for a gun built as a heavy competition gun.
Switching to box lock guns...
The Spanish view box lock guns as having social value inferior to that of side lock guns. Spanish gun makers can, have and still do make expensive box lock guns - but they'd rather not. The upshot of this is Spanish box lock guns tend to be very inexpensive in comparison to Spanish side lock guns.
If you are looking for a field gun, maximum gun value and don't much care about social value, here are some guns to look over:
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=418080242http://www.gunbroker.com/Auction/ViewItem.aspx?Item=415902830http://www.gunbroker.com/Auction/ViewItem.aspx?Item=417386812http://www.gunbroker.com/Auction/ViewItem.aspx?Item=417386812 Did this help, at least a little?