I'm sorry for not specifying my answer enough but there's so little info in the google (i have tortured it for several weeks but with very little success) i thought i won't get any answers if my question was too narrow
On early plate armor:
http://myarmoury.com/feature_mail.html
CATHEDRAL, FORGE, AND WATERWHEEL by Frances & Joseph Gies
During the high medieval times, some "one-piece" helmets were made (e.g. norman helmets, king Wenceslas' helmet) weighing about 2-3 pounds. So making a sizeable plate was not beyond the means of blacksmiths of those days, though most of the early helmets were made of several small plates riveted together (spangenhelm). Romans also were making solid iron helmets during the early imperial period (1-2 centuries A.D.) until they got back to bronze helmets (and iron mail for body protection, back from lorica segmentata).
ThomasPowers, I have scoured through "The Knight and the Blast Furnace" but didn't find what i need. Thanks for the second reference, will look there.
The weight and thickness info i have obtained from several sources. Breastplates were typically 2-4 kg, about 2 mm average thickness, more in the center, less on the edges.
I understand there could be a lot of different factors affecting the price. I don't insist the time to make iron into a sheet was the key factor in the cost of pate armor, i was just guessing if it was so. Now i don't think it was (as it could be done just for 1-2 manhours of unskilled labor). Maybe the forging of the bloom was, though that does not seem likely too..
I've found in "The Knight and the Blast Furnace" the author estimates a 10 kg billet was needed to make a 2.5-4.5 kg plate (so the total time should be increased 2-4x times? or less as the forging melts away and it remains less to hammer). He also writes there's no trace of fold-forging so these large ignots should have been made from a single bloom. But the amount of loss in both phases (bloom to billet, billet to sheet) should depend on the speed of work a lot...