Proof Below. Overpay calculated

Offers

http://www.tf2outpost.com/trade/13055891:

14 keys

7 keys + 6 in metal (13 keys)

16 keys in items

14 keys again

Bubbling Festiver (14) + Sweets

13 keys + 3 in sweets

http://www.tf2outpost.com/trade/13697964 :

13 keys + G. Ap-Sap = ~18 keys (retracted)

Sales

http://www.tf2outpost.com/trade/12495293 - 17 keys

http://www.tf2outpost.com/trade/11138097 - Planets Counterfeit Billy (15 keys) + Unknown sweets

http://www.tf2outpost.com/trade/10928928 - Flies Hound Dog (13 keys). I'd say this is an outlier since recent offers and sales suggest a higher prices, since this was 3 months ago.

Conclusion

High-end based on highest sale, low-end based on an offer and an estimated sale. Either way, I wouldnt think the sale would have reached higher than 17-18 keys, and even if it did reach slightly, this price is more accurate than the current, which is what matters.

Help / Consturctive Criticisim / Additional Proof / Counter-Proof is ALWAYS welcome! :D