No the problem with finding the true costs of Ecoboost engine is that its ONLY found on topend models of what its being installed in.
Seriously, the Taurus Limited Starts at 31K and the SHO starts at 36K...Ford doesn't have the build and price out for the 10 Taurus yet, but I don't think the Limited is going to be lacking any of the options outside of 20in High performance tires and the Ecoboost engine the SHO comes with.
I think that the cost per MPG improved vs V6 will be seen till the Ecoboost I4 engines come along, since the current V6 EB is more or less an Luxury option replacing a V8 engine in these said cars and well yea it gets decent MPG its not the huge improvement that you'll see if u had a EB I4 replacing a V6 in a product, like a 30 MPG Explorer with a 2.5L Ecoboost.