Updating my previous suggestion to give a flat value. Too much hate over 2 keys.

Sold for: 5 buds + 9 keys + Party Trick ~ 5.6 buds 5.5 buds

http://www.tf2outpost.com/trade/19451146 -> Declined many offers, some of which are above the current range.

http://imgur.com/Or659SU Trade history (because apparently, screenshots from the trade can be faked, but this one cannot be)

Only 1 more on the market at the moment.

Feel free to provide additional proof. This is all I could find.