Counter Suggestion.

http://backpack.tf/classifieds/?item=Bill%27s%20Hat&quality=6&tradable=1&craftable=1

1.5 pages of bills below 4.8 keys, some even have paint and spells applied

Sellers/Solds below 5 keys in order. (unpainted)

http://www.tf2outpost.com/trade/23313898 (4 keys)

http://www.tf2outpost.com/trade/23346537 (4 keys)

http://www.tf2outpost.com/trade/23345819 (4.2 keys)

http://www.tf2outpost.com/trade/23374670 (4 keys 3 ref)

http://www.tf2outpost.com/trade/21617523 (4 keys 4 ref)

http://www.tf2outpost.com/trade/23367174 (4 keys 4 ref)

http://www.tf2outpost.com/trade/23373478 (4 keys 4 ref)

http://www.tf2outpost.com/trade/23314409 (4 keys 4 ref)

http://www.tf2outpost.com/trade/23189862 (4 keys 4 ref)

http://www.tf2outpost.com/trade/23316742 (4.5 keys)

http://www.tf2outpost.com/trade/23347229 (4.52 keys)

http://www.tf2outpost.com/trade/23270589 (4 keys 5 ref)

http://www.tf2outpost.com/trade/23373301 (4 keys 5 ref)

http://www.tf2outpost.com/trade/23392169 (4 keys 5 ref)

http://www.tf2outpost.com/trade/23384773 (4 keys 5 ref)

http://www.tf2outpost.com/trade/23272356 (4 keys 6 ref)

http://www.tf2outpost.com/trade/23250056 (4 keys 6 ref)

http://www.tf2outpost.com/trade/23371124 (4 keys 6 ref)

http://www.tf2outpost.com/trade/23353397 (4 keys 6 ref)

http://www.tf2outpost.com/trade/23224664 (4 keys 7 ref)

http://www.tf2outpost.com/trade/23369367 (4 keys 7 ref)

http://www.tf2outpost.com/trade/23281073 (4 keys 7 ref)

http://www.tf2outpost.com/trade/23359103 (4 keys 8 ref)

http://www.tf2outpost.com/trade/23335976 (4 keys 8 ref)

http://www.tf2outpost.com/trade/23284285 (4.8 keys)

Unsolds of 5 keys and above in order. (unpainted)

http://www.tf2outpost.com/trade/23296527 (5 keys)

http://www.tf2outpost.com/trade/23369798 (5 keys)

http://www.tf2outpost.com/trade/23393231 (5 keys)

http://www.tf2outpost.com/trade/23393143 (5 keys)

http://www.tf2outpost.com/trade/23382392 (5 keys)

http://www.tf2outpost.com/trade/23390201 (5 keys)

http://www.tf2outpost.com/trade/23391440 (5 keys)

http://www.tf2outpost.com/trade/23370636 (5 keys)

http://www.tf2outpost.com/trade/23344685 (5 keys)

http://www.tf2outpost.com/trade/23097862 (5 keys)

http://www.tf2outpost.com/trade/23338157 (5 keys)

http://www.tf2outpost.com/trade/23260014 (5 keys)

http://www.tf2outpost.com/trade/23229269 (5 keys)

http://www.tf2outpost.com/trade/23288726 (5 keys)

http://www.tf2outpost.com/trade/20870244 (5 keys)

http://www.tf2outpost.com/trade/23296411 (5 keys)

http://www.tf2outpost.com/trade/23042045 (5 keys)

http://www.tf2outpost.com/trade/23218669 (5 keys)

http://www.tf2outpost.com/trade/23335942 (5 keys)

http://www.tf2outpost.com/trade/22776072 (5 keys)

http://www.tf2outpost.com/trade/23292418 (5 keys)

http://www.tf2outpost.com/trade/23265341 (5 keys)

http://www.tf2outpost.com/trade/23077833 (5 keys)

http://www.tf2outpost.com/trade/23356668 (5 keys)

http://www.tf2outpost.com/trade/23377679 (5 keys)

http://www.tf2outpost.com/trade/23351071 (5 keys)

http://www.tf2outpost.com/trade/23006461 (5 keys)

http://www.tf2outpost.com/trade/23344319 (5 keys)

http://www.tf2outpost.com/trade/23374039 (5 keys)

http://www.tf2outpost.com/trade/23311933 (5 keys)

http://www.tf2outpost.com/trade/23308706 (5 keys)

http://www.tf2outpost.com/trade/23346528 (5 keys)

http://www.tf2outpost.com/trade/23257276 (5 keys)

http://www.tf2outpost.com/trade/23314585 (5 keys)

http://www.tf2outpost.com/trade/23123375 (5 keys)

http://www.tf2outpost.com/trade/23305784 (5 keys)

http://www.tf2outpost.com/trade/23298705 (5 keys)

http://www.tf2outpost.com/trade/23073539 (5 keys)

http://www.tf2outpost.com/trade/23070404 (5 keys)

http://www.tf2outpost.com/trade/23276212 (5 keys)

http://www.tf2outpost.com/trade/23268974 (5 keys)

http://www.tf2outpost.com/trade/23095634 (5 keys)

http://www.tf2outpost.com/trade/23320660 (5 keys)

http://www.tf2outpost.com/trade/23394001 (5 keys)

http://www.tf2outpost.com/trade/23374030 (5 keys 2 ref)

http://www.tf2outpost.com/trade/22972834 (5 keys 3 ref)

http://www.tf2outpost.com/trade/23039471 (5.5 keys)

http://www.tf2outpost.com/trade/22936465 (6 keys)

http://www.tf2outpost.com/trade/23281771 (6 keys)

http://www.tf2outpost.com/trade/22236919 (6 keys)

http://www.tf2outpost.com/trade/23278112 (6 keys)

http://www.tf2outpost.com/trade/22589810 (6 keys)

http://www.tf2outpost.com/trade/22016218 (6 keys)

http://www.tf2outpost.com/trade/23392960 (6 keys)

http://www.tf2outpost.com/trade/23164352 (6 keys)

http://www.tf2outpost.com/trade/23200335 (6 keys)

http://www.tf2outpost.com/trade/22743920 (6 keys)

http://www.tf2outpost.com/trade/22738380 (6 keys 1 ref)

Now for painted bill's selling/sold below 5 keys

http://www.tf2outpost.com/trade/23393149 (4 keys 5 ref) [pink]

http://www.tf2outpost.com/trade/23379804 (4 keys 5 ref) [purple, waterlogged lab coat]

http://www.tf2outpost.com/trade/23390693 (4 keys 5 ref) [216-190-216]

http://www.tf2outpost.com/trade/23387372 (4 keys 6 ref) [lime, Zepheniah's Greed]

http://www.tf2outpost.com/trade/23381575 (4 keys 6 ref) [green]

http://www.tf2outpost.com/trade/23371244 (4 keys 7 ref) [team spirit]

http://www.tf2outpost.com/trade/23385080 (4 keys 8 ref) [team spirit, gold]

http://www.tf2outpost.com/trade/23289740 (4 keys 8 ref) [team spirit]

http://www.tf2outpost.com/trade/23326109 (4 keys 8 ref) [violet]

http://www.tf2outpost.com/trade/23393584 (4 keys 9 ref) [tinge]

http://www.tf2outpost.com/trade/23248742 (4 keys 9 ref) [Zepheniah's Greed]

Conclusion : 4.5-4.8 is probably the fairest price even though there are sellers at 4 but they are uncommon