I mean the very terms "medieval" and "knight" are problematic in their own right as the medieval period is not particularly closely defined and knights can be anything from Hungarian riders to great lords of England and France. The latter would obviously spend vastly more money on their equipment than the former. The same is true for the term "foot soldier" which can be anything from a hastily raised levy of spearmen to dismounted men-at-arms, both are technically medieval foot soldiers but their cost varies a great deal.

With that in mind, say you are a Lord in 15th century England and you which to equip a retainer as a mounted man -at-arms. We assume that this man owns his own weapons but little else. He will need armor and a destrier. Let's assume he has his own courser already. Let's also, just for arguments sake settle for a harness of plates that is not custom made for him but rather made for a man roughly his size and modified to fit. Say you pay 6 pounds stirling for the armor and adjustments. You pay another 5 for the destrier (prices of war horses varied a great deal with time, demand and quality, if this figure is unreasonable I hope someone with more knowledge than me, like /u/waritter, will correct me). Either way, we end up at 11 pounds sterling for horse and armor.

This is the equivalent of one year and three months of labour for a skilled journeyman, half that for the man-at-arms himself. By comparison, a Main Battle Tank costs a whole hell of a lot more. $6M is the equivalent of 100 years of median income in the US, so, by very rough estimates, fielding a single main battle tank in 2018 cost about the equivalent of 80 men-at-arms in the 15th century.

For this case I assumed that you're fielding a normal retainer as a man at arms, not a titled and landed noble. For the man-at-arms you could expect to pay about 1 shilling per day plus food and lodging. I have no idea what the running expense of an active MBT is but I expect it is significantly higher given that you would also need a crew of 3-5, mechanics, infrastructure, fuel, ammo, spare parts etc etc. The crew alone would mean, if we assume that the tank crew member makes about the same as a skilled journeyman did in the 15th century, that wages alone are twice as expensive for the tank compared to a man-at-arms.

EDIT: Pay updated, thank you /u/Rittermeister.

I found this thread from 2 years ago:

https://www.reddit.com/r/AskHistorians/comments/56ccx9/how_much_would_a_suit_of_armour_from_1455_1485/

credit to u/WARitter and u/Dashukta for answers