How many layers at least for proper DDR3 fanout and routing?
Consumer's telecom router achieve to route DDR2 and DDR3 on 4 layers board.

I've worked on several routers, not on design phase, but vast majority of this high-volume products use PCB with only 4 layers. Using 6 layers prompt a lot of discussion about cost and you should demonstrate that you really can't do a board with 4 layers.

One example of this kind of product: 2 DDR2-400 x16 chips.



Another teardown from IHS but pictures are low quality, this box also feature 2 DDR2 chips on a 4 layers PCB: http://electronics360.globalspec.com/article/3410/netgear-super-hub-2-vmdg485-wireless-router-teardown

Are you taking advantage of bit and byte swapping that DDR3 offer ? It can really help layout by avoiding a lot of crossing.

About costs I'm suprised about Kingston, I never saw a product with their DDR chips. In telecom I mostly see Hynix, Nanya, Winbond and sometimes Etron(tech).

Also talking about 1k quantity prices means to me that you are buying through a distributor and not talking with the manufacturer. So taking the time to contact DRAM manufacturer to negociate price is not something to do at the end, because you may be able to have some easier-to-route DRAM chips at the same cost than a not-negociated DRAM.
You won't save on final BOM price, but you'll save on layout complexity/feasibility and maybe EMC.

Edit: you're talking about 2 x32 channels, if the controller's pinout is mixing the two channels you can't achieve this on a 4 layers PCB.
But if the two channel are more or less separated it may be possible, however you may have to sacrifice other signals to allow DDR signals to pass.

LPDDR3 can also help if controller support it.