A few pieces from The Knight and the Blast Furnace.

A 9th century Frankish warrior would pay 12 Solidi (an ox was worth about 2 solidi) for his mail armour and another 12 for the rest of his arms (helmet, shield, sword and spear).

An early 14th century Flemish knight would pay 10-20 pounds (60-120 days wages for a craftsman) for mail armour, depending on length.

15th century plate armour seemed to start at about £5 (about 100 days wages) in England, with better sets costing more (one being priced at £8 6/8d for a 'professional soldier). These are imported from Italy.

Prices varied wildly based on quality. A common soldier's armour in 16th century Spain was only about 5 Florins, but royalty wore armour costing forty times that. At this time an average wage was apparently about 5 florins a month and an ox cost around 15 florins.