Mr. Harden,
First of all, an honest appraisal should be based on a visual inspection of the diary itself. And you should probably know that the (actual or insured) "value" of your civil war diary will depend on many things. The soldier's service record as well as his regiment's history are two items to consider. The contents found in his daily entries will also affect it's value, especially if it has "battle content" and other significant military information, such as references to commanding officers, details concerning the enemy, as well as any other historical facts that might make the diary unique. These diary entries, rather than those sharing the standard routines of camp life, weather reports, and "longings for home" are more often what appeal to collectors and historians.
Besides the content itself, there are other factors to consider, such as whether the writing is in pencil or in ink, and is the soldier's writing legible (or will it be hard to transcribe)? How often did he write an entry or are there a lot of blank dates? The soldiers attention to detail and his education also contribute to the diaries appeal and value.
And then there is the actual condition of the diary itself to take into account. Is there any damage to the diary such as torn or missing pages, staining, faded handwriting, missing covers, etc.? And is it personally identified by the soldier?
This is some, but not all, of the criteria that would be used to determine the value of your "authentic civil war diary" when appraised by either a historian, a Civil War collectables dealer, or an antiques appraiser. And yet you're apt to find considerable diferences in the appraised value given even between them.
As a collector and historian of Fourth Michigan Infantry items, I have paid two to three thousand dollars each for some of the diaries from soldiers in that regiment. Those particular diaries were worth that much to me, but yet others would disagree. But I have seen many other diaries from civil war soldiers that were priced much less with some as little as two hundred dollars. Many of the concerns that I've mentioned previously very likely were used to determine those values.
So to some it all up, it would really be unfair for anyone among us to to try to give a blind appraisal to something they haven't actually seen, read, and inspected. And I doubt that there are many insurance companies that would accept a "message board" appraisal anyway. Don't take that wrong, while there are many knowledgeable and helpful people on this message board, I'm sure the insurance company will expect something more than that to determine the insured value of your diary. So I would suggest that you have someone reputable visually check your diary out. If you decide to have that done, I'm sure someone on this message board could offer some suggestions as to whom.
I apologize for the length of this reply, but ultimately I hope it helps....Good luck, George Wilkinson