That's easy. More impressive stuff on it means more expensive.

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Role with the Punches...
Elan: A 10% chance is pretty unlikely, but everyone knows that a one-in-a-million chance is a sure thing!

Something to bear in mind is that the total cost for an item isn't a simple addition of the costs for each property, there is multiplication too.

However, the gold piece value of the best item you can equip at your level also increases at a rate better than linear as you gain more levels.

Very useful. That should be stickied somewhere under the heading of "If you mess with Mythals, this could happen to you:"