Lorica hamata / segmentata

[Dan Howard]

How on earth is 12 cows an inconsequential cost? It required the labour of dozens of Carolingian farmers to be able to outfit a single warrior in the above-mentioned equipment (only those with landholdings in excess of 300 acres were expected to be fully equipped in the above manner).

And the above cite isn't entirely accurate. The original source (Lex Ribuaria, V, 36, 11) actually lists the items' cost in currency (solidi), not cows.
helmet 6 solidi
brunia 12
sword 3
lance and shield 2
warhorse 12 each (he had spares)
the other entry probably translates as mail leggings, not greaves (which would be much cheaper than 6 solidi)

Alternative sources give comparative costs for animals
horse (mare) 3 solidi
ox 2
cow 1 to 3

Consider that a fully trained warhorse cost 12 solidi and mail armour (brunia and leggings) cost 18, and you start to appreciate the true worth of mail armour.