We're using an old listing which has since been removed. If someone creates a new listing, the price will be updated.
Prof KS Mad Milk is a glitched item anyway.
So on some items like a Professional Killstreak Mad Milk, the backpack.tf price suggestion is 61.74 keys which is based on a steam market listing which doesn't even exist. Here are the links:
Steam Market listing that doesn't exist but is used as a suggestion: http://steamcommunity.com/market/listings/440/Strange%20Professional%20Killstreak%20Mad%20Milk
and strange professional mad milk buying price: http://backpack.tf/classifieds?item=Mad%20Milk&quality=11&tradable=1&craftable=1
To see the steam market suggestion just click on the strange pro is madmilk buy order at the top and it says 61.74 keys. This is on the 2nd link.
P.S. I had trouble formatting this.
Please fix this as it can hinder trading.