What would the highest possible value for a non-planepacked artefact be? It would have to be an adamantine large serrated disc or giant spiked ball - 300(adamantine)x120(artefact)x126(LSD/GSB) = 4.536.000☼ plus decorations. You get one decoration per material in the artefact, which is limited to base material plus seven for non-planepacked artefacts. Adamantine metal artefacts always (i think) use a single wafer as base, so you get a "free" 360.000 decoration for the base bar. For the other seven, the most valuable choices depend on item category chosen:
metal bar - adamantine (300x)
stone, stone block, cut gem - raw adamantine (250x)
rough gem - coloured diamond or star ruby/sapphire (60x)
bone, leather - roc (15x?)
rough glass - crystal (10x, only possible if you've already made crystal glass items in your fort)
cloth - most valuable choice available for decoration is silk - giant cave spider gives 4x; adamantine cloth is not used for artefact decorations, only as base material
wood - any (1x)
So the ideal would call for seven decoration materials, mostly of types where adamantine is an option. I don't think you can realistically get a smithing mood that asks for seven metal bars for decoration purposes(x), but an extra two bars and four stone/block/cabochon might be possible, with roc leather for the last slot; something around six and a half million dwarfbucks should be just barely possible.
(x) that'd make an actual eight adamantine decorations @ 360.000 each, so... 7.416.000 theoretically? Not sure, i just calculated this in my head.