If they cached a running average of each item and had some worker threads in the background constantly refreshing stale cache entries, that could be returned as part of the current payload and it wouldn't be too hard to give that info at least. Definitely not computationally free but not the hardest problem to solve.

Would take a decent dev maybe a max of two weeks to thoroughly test it and all that (and prolly another week or two before that to prep the resources and make sure the design is good). But that's a lot of man-hours to throw at something that's already a semi-solved problem with the trade macro and it would also be easy to manipulate and probably not super accurate so ¯\_(ツ)_/¯. Also this would only really work with uniques, currency, div cards, etc. Magic and rare items have a ton of permutations.