A pony hat... in TF2? Even after they made fun of ponies last Halloween? *sigh* I just don't know anymore. Anyways, this is the proof I've mocked up from any sources available. Side note: Any time I haven't said out the name, the abbreviation will be MM. (P.S. Ghost Trick + PW = Best Capcom games ever (except if you're a Megaman fan)).

CLASSIFIEDS:

http://backpack.tf/classifieds/search/id/527043f54dd7b8fb108b4567 - Sellers at 2 keys, 3 keys, 9 keys, and 10 keys, although the majority are around 2 keys. Both 2 key sellers seem to have sold already.

SELLING:

http://www.tf2outpost.com/trade/15394700 - Offered many things for the Magical Mercenary, one being 1 key + a Unique Magical Mercenary for the Haunted Magical Mercenary. Also offered 2 Spooky keys + .5 in items, but the user hid their post.

http://www.tf2outpost.com/trade/15372947 - Selling for 4 keys. Offered a crap ton of items for it, and the owner was willing to accept it. Also offered 4 keys in full. This trade might be a possible outlier.

http://www.tf2outpost.com/trade/15403734 - Selling his for 4 keys and he gives 1 ref back (~3 keys, 5 ref). Has had one add, but hasn't sold.

http://www.tf2outpost.com/trade/15404504 - Selling his for offers. Has gotten offers of around 1.5 keys (none of which he's taken yet, thankfully).

http://www.tf2outpost.com/trade/15368758 - Offered ~2 keys, Spooky or non, for the MM.

http://www.tf2outpost.com/trade/15412733 - Rejected 1 Spooky Key for the MM.

http://www.tf2outpost.com/trade/15416753 - Selling his MM for 2 keys.

http://www.tf2outpost.com/trade/15368386 - Offered 3 keys (which is ~2.2 keys for the hat itself) and a S. Frying Pan (more then likely an outlier).

http://www.tf2outpost.com/trade/15421308 - Selling for 2 keys + 1 ref. Unsold, 4 hrs old, and never bumped.

http://www.tf2outpost.com/trade/15419705 - Selling for 3 Spooky Keys each, but has gotten no adds. 5 hrs old.

http://www.tf2outpost.com/trade/15416113 - Selling his for 3 keys flat, but has gotten no adds. Again, less than a day old.

http://www.tf2outpost.com/trade/15410849 - Selling his MM w/ a Gold V. Baker Boy (~7 ref) + 2 keys for a Bill's (making his MM around 3 keys).

http://www.tf2outpost.com/trade/15422731 - Offered Pink Paint (1 key) + "some" 1.66 hats.

http://www.tf2outpost.com/trade/15398114 - Wants 4 keys for his brown-painted MM, but no one's bought it yet.

http://www.tf2outpost.com/trade/15424634 - Wants 4 keys for his MM.

http://www.tf2outpost.com/trade/15415448 - Yeah, I don't know about this one, since none of these have prices.

http://www.tf2outpost.com/trade/15416195 - Selling for 3 keys. Hasn't sold, and less than a day old.

http://www.tf2outpost.com/trade/15378603 - Offered 2 keys for his MM.

http://puu.sh/565ZT.png - I personally bought it for 2 keys, because y'know, humiliation by a pink pony hat (and I love the voice lines).

SOLDS:

http://www.tf2outpost.com/trade/15418272 - Wants 3 keys for his. Seems to have sold to Solaris.

http://www.tf2outpost.com/trade/15400140 - Sold for 2 keys + 3 ref according to the history.

http://www.tf2outpost.com/trade/15403761 - Sold 3 for 2 keys each in less than a day.

http://www.tf2outpost.com/trade/15378546 - Seems to have sold his for 2 keys.

http://www.tf2outpost.com/trade/15381935 - Sold for 2 keys.

http://imgur.com/vwNPhoo - Sold for 3 keys.

BUYERS:

http://www.tf2outpost.com/trade/15425874 - Sellers wants 2 keys for his, but the trade owner hid his post.

http://www.tf2outpost.com/trade/15359015 - Wanting to do a 2:1.

http://www.tf2outpost.com/trade/15389408 - Another 2:1.

http://www.tf2outpost.com/trade/15396827 - 2many2:1's4me

The rest of the buyers are the same, offering lower or doing 2:1's of items that aren't priced.

Conclusion: Too many people on the market, I swear. Anyways, most people seem to be able to sell around 2-3 keys, so I'm just going to put that up as a starting price for now. If you find any proof/counterproof you'd like to contribute, please do so in the comments section. Thanks for your consideration.