Not trying to nitpick, but the term "diamond grade" is not correct. The reason that is important is that various models sold under the "diamond quality" banner differed significantly in price. Canvasback's diamond quality model 275 above, while really gorgeous, would sell (then and today) for less money than a similar condition Daly diamond quality model 375 for example. My point is, all other things being equal, I'd expect the model 375 to sell for 25% - 75% more than the model 375, even though they are both a "diamond quality" guns and marked as such on the ribs.
In terms of what can I expect to pay for a Lindner Prussian Daly diamond quality shotgun, I think your best resource is the result libraries of auction companies like James Julia and Rock Island. The libraries of past auction results are free to browse once you register. Like the rest of the vintage shotgun economic landscape, you'll see that the prices of the Daly guns vary tremendously due to things condition, gauge, dimensions, features, etc.
Due to that variability, it is difficult to put a price on what you can expect to pay. It depends on a lot of variables.
FWIW, I bought a restocked Lindner Daly diamond quality hammerless 10 for $1800 (about 10 years ago) and lost out on another (in much, much nicer condition) that went for over $8000.
