Countering both suggestions: http://backpack.tf/vote/id/55570fe9b98d88dc158b46ea & http://backpack.tf/vote/id/5559e22bdea9e9074b8b4569

Conservative drop.

Classified ads at time of posting:

http://backpack.tf/classifieds/?item=Bill's%20Hat&quality=3&tradable=1&craftable=1

5*12 keys (2 are painted Team Spirit and 1 is painted Black)

6*13 keys (2 are painted Team Spirit, 1 is painted Pink, 1 is painted Greed and 1 is painted Slate)

Sellers at 13 keys:

http://www.tf2outpost.com/trade/25467635 (Duped, painted Black, ~2 months old)

http://www.tf2outpost.com/trade/25898946 (Painted pink, ~2 weeks old)

http://www.tf2outpost.com/trade/25996621 (~2 days old, poorly bumped)

Seller at 12 keys:

http://www.tf2outpost.com/trade/25986724 (~3 days old)

I believe this is an accurate representation of the current market for the Vintage Bill's Hat as 13 keys is still a common trading point as seen in the classified ads and current Outpost trades for this item.

This is all I could find on Outpost that are valid at the time of posting this suggestion.