13-14th century English Knights fought on foot because of tactics not because they didn't have horses.
The 2 main battles from the era you specify are Crecy and Poitiers. Both consisted of the English defending a hill of some sort with inferior numbers and in both cases it therefore made sense for Knights to fight on foot. Fighting on foot means you are better able to hold ground and are less likely to run away. At Poitiers however some of the English knights remained mounted allowing them to turn the French flank and capture the French King to end the battle.
As for Plate armour, the average plate armour was relatively cheap in reality. It took less time to make than chain mail, had no moving joints and was full of imperfections that made it vulnerable. Overall it was probably the cheaper part of most knight's armour since most of them would have been wearing a chain mail shirt under their plate anyway.
Obviously good plate was much more expensive and as such it was much rarer
take a look at this for accurate pricing...
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loginhere is a link to that page, also with listed currencies.
http://www.luminarium.org/medlit/medprice.htmas you can see, maximum price of warhorse was L80 [in 13th century, there is no info that armor was included, but since cost is this high, ill take it as granted that we are talking about armored war horse. cost in shillings is 1600]
whereas battle-ready full milanese armor was about 166 shillings and 8 deniers. [1441]
however, unarmored war horse was worth 50 shillings in 12th century.
so to certain degree
kafein was right about plate armor being expensive, but not as much as millions of crpg gold.
id wager that it would be about 40-50 k of crpg gold if devs were going for realism...