Proof below (4 sales). Overpay is applied.

8 month-old price.

Prices used are of time of sale.

1st sale:

http://www.tf2outpost.com/trade/13692627 mc22 sold this for nnb cosa+8 keys=23 keys

2nd sale:

http://www.tf2outpost.com/trade/12647570 sold for 21 keys+3 ref=21.5 keys

3rd sale:

http://www.tf2outpost.com/trade/12989795 sold for of beret+2 keys=~25 keys, seems like outlier especially since he was only looking for 19 keys pure or 23 keys in unusuals

4th sale:

http://www.tf2outpost.com/trade/14442741 added 1 key to sell 1 bud=20 keys

Sales: 20, 21,5, 23, 25

The 25 keys sale seems like an outlier, so I'm not including it in the range. I've tried my best not to use the inflated unusual prices here.