You may be interested in this previous answer from me. That being said, looking at the Howard accounts and talking to Tobias Capwell at the Wallace collection I may need to revise my low estimate down to only 2-3 pounds or event less for certain 'off the rack' imported armours, but this is less well attested in the secondary literature I have on hand. Note that this is still several months wages for a skilled workman. Regarding your timeframe, while the cost of infantry armours goes down in the 16th century, the cost of a full 'knightly' harness (set) of armour remains relatively steady at several months to a years wages for a skilled workman or soldier.

As to your question about buying a full harness all at once versus in pieces, in the 15th century full armours seem to often be bought as full harnesses. -However- we also see armours that are bought complete 'except for the greaves' or 'except for the helmet' - probably because someone already owned these. We also see only parts of armour being issued out by a lord to loan to his retainers, but given the context of these records (naval expeditions and border skirmishes) I think this may represent a 'light' armour that was more suited for skirmishing and other types of irregular warfare rather than pieces of a full 'cap a pie' armour. What we don't have though is a record of someone buying an armour piece by piece and slowly building up to a full harness - often by the time people went in to buy armours they went 'all in'.