It costs $20,000 to use this weapon for 12 seconds.

Classified ads at the time of posting:

Page 1 - http://m.imgur.com/hahenPB

Page 2 - http://m.imgur.com/8KmKk0G

Page 3 - http://m.imgur.com/JVUX8ab

http://backpack.tf/classifieds?item=Minigun&quality=11&tradable=1&craftable=1&australium=1

3*18 keys

1*18.52 keys (Has Killstreak)

8*19 keys (4 have Specialized Killstreak, 1 has Killstreak)

1*19.31 keys

9*20 keys (6 have Specialized Killstreak, 3 have parts attached)

1 buy order at 18 keys (Limit of 1)

Unsolds at 20 keys:

http://www.tf2outpost.com/trade/27516685 (~2 weeks)

http://www.tf2outpost.com/trade/27519576 (Has Specialized Killstreak, ~2 weeks)

http://www.tf2outpost.com/trade/27559854 (Has Specialized Killstreak, ~4 days)

http://www.tf2outpost.com/trade/27350437 (~1 month)

http://www.tf2outpost.com/trade/27317321 (~1 month)

http://www.tf2outpost.com/trade/27536276 (Has Specialized Killstreak, ~1 week)

http://www.tf2outpost.com/trade/27568803 (Has Specialized Killstreak, ~1 day, young)

http://www.tf2outpost.com/trade/27463516 (Has Specialized Killstreak and Buildings Destroyed, ~3 weeks)

http://www.tf2outpost.com/trade/27499277 (~2 weeks)

http://www.tf2outpost.com/trade/27232049 (Has Specialized Killstreak, ~2 months)

http://www.tf2outpost.com/trade/27518639 (~2 weeks)

http://www.tf2outpost.com/trade/27469662 (~3 weeks, poorly bumped)

http://www.tf2outpost.com/trade/27482916 (Has Specialized Killstreak, ~2 weeks, poorly bumped)

Unsold at 19 keys and 6 refined (~19.3 keys):

http://www.tf2outpost.com/trade/27568794 (~1 day, young)

Unsold at 19 keys:

http://www.tf2outpost.com/trade/27555467 (Has Specialized Killstreak, ~5 days, never bumped)

Unsold at 18 keys and 9 refined (~18.5 keys):

http://www.tf2outpost.com/trade/27507815 (Has Killstreak, ~2 weeks)

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