@Macschrauber is talking about early-2009 CPU trays, you are talking about mid-2010/mid-2012.
Early-2009 dual CPU trays are a lot cheaper than mid-2010/mid-2012. There are a lot of early-2009 dual CPU Mac Pros, not so much mid-2010/mid-2012, since they sold a lot less numbers and hex CPUs were already available at the time.
Yeah, in Europe Single starts at 100 EUR. Dual not under 500 regardless whether 4,1 or 5,1. Shipping from US is most times too expensive.
So for now, I will keep it single-6core and looking out for a miracle...
Thank you all for helping!
...so now I have to figure out what to do with my new "Home"-Enterprise-Server rack with the dual 3,46 GHz.
(everything cleaned, updated and upgraded and complete with cablearm – If only the fans weren't so loud)