Not a huge demand for these. There are plenty of people selling for 2 or 3 keys but the trades aren't being sold. 5 ref seems like a decent starting point for this item.

Unsold for 2 keys

http://www.tf2outpost.com/trade/7591688 - Unsold for 2 keys

http://www.tf2outpost.com/trade/7643914 - Unsold for 2 keys

http://www.tf2outpost.com/trade/7643856 - Unsold for 2 keys

Sold/Offer of 5 ref

http://www.tf2outpost.com/trade/7625830 - Sold for 5 ref

http://www.tf2outpost.com/trade/7580052 - C/O of 5 ref

http://www.tf2outpost.com/trade/7481850 - Offer of 1 key + 2 ref

It's not much, I know, but it's all that OP could tell me. 5 ref for now, then, and we'll see what the market tells us.