I was lucky and got one for 2.11 ref recently (the person who sold it to me was the original owner, and didn't own a set). It is hard to price because haunteds have a small number of buyers and sellers on outpost. The people asking for 2 keys for a haunted scout set don't sell quickly in my opinion, or even get offers of 4+ refined. I think only the most desperate buyer would get a set for more than 2 keys.

http://www.tf2outpost.com/trade/4708376 (4 days since last bump)

http://www.tf2outpost.com/trade/4910184 (asking for 5 ref)

That said though people might be compelled to buy for more, as it is a relatively uncommon item sold individually.

http://www.tf2outpost.com/trade/4918163 (buying for 1 key)

Oh sorry the first link is misleading, the guy did sell it but hasn't closed his trade. Maybe this suggestion is still accurate.

Well both those 2 key sets did sell. I'll upvote this.