Edit: I never check if a suggestion is up for an item, didn't mean to counter.

Sellers:

http://www.tf2outpost.com/trade/27385191 - 25 Keys x2 Spec KS - 3 Days

http://www.tf2outpost.com/trade/27321088 - 25 Keys - 2 Weeks

http://www.tf2outpost.com/trade/27373844 - 25 Keys - 4 Days

http://www.tf2outpost.com/trade/27344253 - 25 Keys - 10 Days

http://www.tf2outpost.com/trade/27348016 - 25 Keys - 10 Days

http://www.tf2outpost.com/trade/27364232 - 25 Keys - 6 Days

http://www.tf2outpost.com/trade/27371341 - 25 Keys - 5 Days

http://www.tf2outpost.com/trade/27380497 - 25 Keys x2 - 3 Days

http://www.tf2outpost.com/trade/27389688 - 25 Keys x3 (2 Spec KS) - 2 Days (jeez puddington ;-;)

http://www.tf2outpost.com/trade/27394084 - 25 Keys - 2 Days

http://www.tf2outpost.com/trade/27396777 - 25 Keys - 1 Day

http://www.tf2outpost.com/trade/27391516 - 24 Keys 2 Ref (Spec KS) - 2 Days

http://www.tf2outpost.com/trade/27387648 - 24 Keys (Spec KS) - 3 Days

http://www.tf2outpost.com/trade/27395093 - 24 Keys (Spec KS) - 2 Days

http://www.tf2outpost.com/trade/27396749 - 24 Keys (Spec KS) - 1 Day

http://www.tf2outpost.com/trade/27367626 - 24 Keys x2 - 6 Days

http://www.tf2outpost.com/trade/27381929 - 24 Keys - 3 Days

http://www.tf2outpost.com/trade/27394151 - 24 Keys - 2 Days

http://www.tf2outpost.com/trade/27401202 - 24 Keys - 1 Day

http://www.tf2outpost.com/trade/27400820 - 24 Keys - 1 Day

http://www.tf2outpost.com/trade/27401658 - 23 Keys 11 Ref - Young

http://www.tf2outpost.com/trade/27404201 - 23 Keys - Young

Classifieds:

http://imgur.com/a/ILVwp

x2 23 Keys 14 Ref

x2 23 Keys 15 Ref

x1 23 Keys 16 Ref

x10 24 Keys (x4 Basic KS/x2 Spec KS)

x1 24 Keys 2 Ref (Spec KS)

x2 24 Keys 8 Ref (Both Spec KS)

x11 25 Keys (x5 Spec KS)

Conclusion:

25 Keys can't sell and there are a bunch of unsolds at 24 Keys as well. 24 Keys seems to be the most common trading point, taking a flat range of 24 Keys.