As a professional Google page turner/searcher, a historical enthusiastic cultic college student with no flavor of degree, of stubborn opinions, and most starving of 3D artists, I conclude from my observations of reading about Brunea is that it is a vague term for armour required for Carolingian aristocrats to wear for military campaigns. Brunea is also considered awesome loot for Vikings besides Frankish swords. Whether something was considered expensive during the medieval timeperiods is all based upon agreed and shared views of many individual observations that could easily be proven wrong if a text or two of specific descriptions and statistics gets discovered. I would also like to mention that the majority of people didn't follow a set monetary system and if that were the case, prices could rise and drop. I have read a few books (like 2 or 3) of authors, and read through several online places of cults, forums, and research articles that define brunea as either scale,
chainmail, both, or armour in general. I would only refer to brunea as a carolingian word for body armour because that is the most acceptable and truthful of answers. I would highly recommend to understand brunea as a very flexible and broad word for body armour created during the Carolingian time period unless you want to argue with people that have biased views. The illustrations (frescoes, bible drawings) that depict the Carolingian helmet and other various forms of armour can be interpreted by anyone in the past viewing those same drawings. There was no standard of industry during the medieval era that created one kind of armour. The popular interpretation of the brunea and the carolingian helmet is not impossible for someone in the past to interpret it similarly. School textbooks in the United States and other similar sources describe the Carolingians and their empire as a revival of Roman civilization (which anyone and everyone could argue for days about).
Short answer is that you can neither claim nor disclaim whether brunea was defined as scale unless a found and trusted archaeological statistical evidence says otherwise. If the scale armour is depicted in drawings, it could have existed.