My last suggestion was well received, but the range was wrong. So I'm reopening it with a more accurate range, as well as a bit more proof.

http://backpack.tf/vote/id/51589226ba2536c928000003

Judging by my proof + everyone's contributions, we determined that yes, it is possible to sell for 2 keys, but there are also many unsold for 2 keys as well as the occasional seller for lower.

Now then, here are some sellers unsold that are under 2 keys.

http://www.tf2outpost.com/trade/9710762 - Key + 4 ref (Unsold for 2 days despite bumping)

http://www.tf2outpost.com/trade/9701135 - Key + 3 ref (That's about 1.75 keys, unable to sell at key + 4 so they lowered the price, still no takers)

I was unable to find any new successful buyers, but not many new trades opened up in the 2 day window. I'll post them if I find any.

http://backpack.tf/vote/id/5154b0b6ba25361744000001 is still pretty relevant, out of his 10 unsold proofs, only 2 have sold to this day (a few more were closed, but the item's still in their bp)