It’s an interesting thought. But dealing with artefacts (not magical ones) in my profession I agree that you can’t put even a Guide price in print. The word “priceless” exists for a reason. You can’t total the value of the materials and labour as you can with say, a new piece of armour. In fact a magical artefact is effectively worthless, it has been “removed from the economic cycle” - until someone wants it. Then it’s worth exactly what they are prepared to pay for it. That’s why auction houses exist - to test the demand and discover how much people are prepared to pay.
Which of course makes for a possible adventure. The PCs touting their artefact around a number of potential buyers, playing them off against each other (and hoping none of them decide its simpler just to have some henchmen kill the PCs).
And the answer to what the final price is? How much money do you want the PCs to have?
I do agree and understand that the value is based on how much others want the items, and the history and use of the items will affect the final price. As I get more familiar with the economy of Fbl. I am sure that gauging this will become easier, I will look through the treasure tables to see what is considered a valuable find and how much coin it is listed as to find a starting point.
At this stage my players are not looking for riches, just enough coin to survive day to day, as they can't even afford a meal and a bed in the Inn, they spent their last coin on a shovel to dig up a burial cairn they found which turned out to house a death knight and a pretty necklace (valued at 4 shovels!).
So any bauble they find is likely to be valued in practical worth unless it will give them a definate and useful boost but what they will find useful is anyone's guess.
And further down the line they will be looking to amass as much gold as possible to build the fortress they have dreamed about. If they aren't interested in uncovering the main plot then they will look to trade valuable items to be able build the next function and improve thier new home. And if that's what they want to do I will need to find a way to answer thier simple question of "how much is this thing worth?" (One of the characters is a pedlar so he has spent his life asking that question). Finding a buyer will be something I will definately have fun with though.