6 month old price!

Sale 1:

Sold for Duped Green Energy Doctor's Sack* (~28) + 24.91 USD in SCM (~10) = 38 keys

http://www.tf2outpost.com/trade/27043425

http://backpack.tf/profiles/76561198044270517?time=1442300400

http://backpack.tf/profiles/76561198044270517?time=1442538774

http://backpack.tf/profiles/76561198044270517?time=1442538774&compare=1442300400

http://i.imgur.com/3Ge6H9W.png | http://i.imgur.com/wWCxjRJ.png

Sale 2:

Sold with Specialised Australium Minigun for Knifestorm Hat With No Name...

http://www.tf2outpost.com/trade/26996845

http://backpack.tf/profiles/76561198103060345?time=1441954800

http://backpack.tf/profiles/76561198103060345?time=1442127600

http://backpack.tf/profiles/76561198103060345?time=1441954800&compare=1442127600

Sale 3: (outlier)

Sold with Specialised Australium Minigun (~22) + Sweets (~5.65) for Professional Australium Scattergun (~45) = ~17 keys

http://backpack.tf/profiles/76561198058198301?time=1442214000

http://backpack.tf/profiles/76561198058198301?time=1442386800

http://backpack.tf/profiles/76561198058198301?time=1442214000&compare=1442386800

Mini-suggestion for Duped Green Energy Doctor's Sack*:

Sold for Australium Medigun (~28) + Sweets (~1.7*0.9) = 29.55 (B/O caps it at 28 keys)

http://www.tf2outpost.com/trade/27025110

http://backpack.tf/profiles/76561198044270517?time=1442127600

http://backpack.tf/profiles/76561198044270517?time=1442214000

http://backpack.tf/profiles/76561198044270517?time=1442214000&compare=1442127600

Notes:

These are the only recent sales that have occurred within 2 months.

Even if Sale 2 is an outlier, it clearly indicates that the Ze Goggles was valued at more than the seller's B/O of 40 keys.

http://www.tf2outpost.com/trade/26996845 - B/O of 40 keys

http://www.tf2outpost.com/trade/27043425 - B/O of 40 keys

Conclusion:

Sales at 38, >40, and 17 keys. Taking a final range of 38-40 keys.

Feel free to support/counter my suggestion!