I wrote a response a little while ago about roughly how much armour would have cost during the High Middle Ages here: http://www.reddit.com/r/AskHistorians/comments/26nuj9/how_expensive_were_medieval_weapons_and_armor/

Prices understandably fluctuated a lot in the Middle Ages and my response doesn't cover Plate Armour which would be hugely more expensive. I'd have to do some digging to find out exactly how much more Plate Armour was.

For converting the cost into modern money, that's tricky and really not my area of expertise. Our economy doesn't really line up with the medieval economy so there's no perfect way of doing it and estimates can vary wildly. I've seen some people do some pretty good explanations and estimates here, though, so hopefully one of them can come along and update my numbers since I'm not qualified to do it.