Ok, easy to find on the classified for 4 ref - 4.33 maybe not so easy on outpost seeing as it is filled with people who overprice and lowball

Evidence:

http://backpack.tf/classifieds/search/founding%20father You can find quite a few for 4-4.33 also there is one listed for 3.88.

http://www.tf2outpost.com/trade/12946466#p41752143 - 3.33

http://www.tf2outpost.com/trade/12894356 - 4.33

http://www.tf2outpost.com/trade/12812552 - Key Unsold After 4 Days

http://www.tf2outpost.com/trade/12891405 - 5.66 Unsold After 1 Day

Just got my one for an FDU (Fancy Dress Uniform valued at 3.33) http://www.tf2outpost.com/trade/12946466

http://www.tf2outpost.com/trade/12834254 - Selling for 4 ref, was originally selling for 5 I think.

Couldn't find any buyers on outpost because people are lowballers :3

This is some of the better evidence I could find so, if you have anymore, please put it in the comments below.