The only camera anywhere close to MP & price will be the 645Z - and that body is $8,500. No reason to expect the 5Ds to be less than the 1Dx. Should be between the two.
Why? Because they can, and there isn't a reason to not. I am willing to bet that Nikon wishes they'd kept the D800 price a bit higher so they could discount it as needed to maintain sales numbers.
We're all speculating here, so to each his/her own...
That said, in my opinion, the decision to but a 5D badge on this body immediately constrained it to a particular price ceiling. To my knowledge, Canon doesn't even officially refer to the 5D Mark III as a "professional" body -- that honor is reserved for the 1 Series.
In terms of "because they can," that would only work if Canon held a monopoly. Competition (especially from upcoming high-megapixel Sony sensors) and other market forces will produce an equilibrium price that balances what Canon asks with what the market will support.
The only question in my mind is, "What is the 5D series price ceiling?" There is precedent for increase (5DII to 5DIII), so it's possible to see a similar bump. Timing and market conditions (both DSLR and financial markets) seem to point to a modest bump at most, but probably not huge -- unless this body blows everyone's minds. In which case, the target market will be lining up demanding that Canon take their money.
One other quick observation on pricing theory: It's a lot easier to start higher and lower the price than start below what the market will bear and increase the price.
People tend to get a lot angrier over price increases
than high initial MSRPs. If a company underestimates what the market will bear and intros at too low a price, that profit is left on the table -- not good business strategy.
Even if Canon could perfectly predict what the market will bear, starting higher is better strategy. Early adopters and "must-havers" will pay the premium. Subsequent price drops have a positive effect on the psyche of the target market (like seeing a sofa with a price tag that says, "