Hi David,
some great thoughts... but I dont think it is ever possible to have a one size fits all solution. Each collector can work out what he considers to be criteria for "premium prices", but it will always remain subjective because there are many collectors who collect with special interests.
For instance, while I agree with much of your scale above, I put little value on a medal being mint. I would also value infantry over cavalry, especially for WW1.
My major collecting area is WW1 German award certificates... here prices are usually determined by the design on the document, with certain interest groups assuring that documents for certain battles (verdun, hartmannsweilerkopf etc) fetch premium, and certain units fetch a premium.
Here is a fine example (for me)... The DSO group... Officer with Danie Theron, sharpshooter, raider in the Cape, documented shootings in a number of books, etc. etc...
http://gmic.co.uk/index.php/topic/3093-for-killing-brits/?hl=cloete#entry27693
For me, the MYB can go suck an egg with valuation... if I for some reason wanted t sell it, I would put EUR15 000 on it... it is for me the ultimate Boer Group, the only Boer Sharpshooter with so much description of his actions... and I would probably not get EUR15 000 for it because other collectors have other priorities... so? What is the value of the group? I contend, as long as it is in my hands, and I am alive and kicking... the Value IS EUR15 000... when I die and my wife decides to ebay it... the value drops to whatever the next 2 collectors decide to bid on it... With unique things like named medals the value is not only set by what someone is willing to pay... but also by the lowest price a seller is willing to accept.... even if noone meets the price...
This is a great can of worms and i see room for lots of interesting discussion :-)