Bought it for 384 keys, 2 professional killstreak strange australium sniper rifles and 2 strange australium eyelanders (384 + 58 + 58 + 82 + 82) = 666 keys

https://backpack.tf/profiles/76561198162120088#!/compare/NaN/1643241600

Sniper buyers at 58.17 keys https://backpack.tf/classifieds?item=Sniper%20Rifle&quality=11&tradable=1&craftable=1&australium=1&killstreak_tier=3

Eyelander buyers at 82.9 keys https://backpack.tf/classifieds?item=Eyelander&quality=11&tradable=1&craftable=1&australium=1&killstreak_tier=0

Rounding everything to 666 keys because it is a nice number and rounding down is almost always better for suggestions such as this