I agree with Chris' perspective on this... The value is purely subjective to the owner regardless of any formula or value guidebook. And when one factors in "market forces," clearly, what one paid for an item in the first place (value determined by someone else) plays a part in any re-sale equation. And as Chris states, value is to a large degree what one is willing to accept for the item in a sale transaction after adding in subjective criteria. I may value an item high, but you may value it low - simply because we have different interests. It seems more voodoo than pure math.
Frankly, to play the devil's advocate, any piece has only subjective value until it goes to market. Unless you can get said price, the item is either "priceless" or "worthless" - depending upon your perspective. Anything outside my collecting field simply has no "value" to me...since I'm not willing to pay any price. Sort of like a bloody sock. But hey, to another collector it's worth thousands of dollars because of the subjective factors such as it's history and the owner's attachment to that history.
For insurance purposes, one is at the mercy of random forces it seems... Realized market prices and guide books. Appraisers tend to be less attached to these historical items than are we...
I boil it down simply; it's "value" to me is whatever I paid for the item to obtain it in the first place. Should it go to sale, it's value is whatever I'm willing to accept for it at the time. To sell, I either lower my "value" to the buyer's or get the buyer to raise his "value" to match my price. Simple supply and demand.