When I was talking about a heat shield designed for a Mars return being able to handle a lunar return, I was talking about the heat loads. If it can survive a 13 km/sec reentry heat load, then an 11 km/sec heat load shouldn't be hard to handle.

I'm not certain of the percentages, but I'd WAG that perhaps 20% of the cost of developing a manned spacecraft is in the structure and heat shield. I may be wrong on that, but aerospace development trends indicate that things like software development are an increasingly expensive portion of R&D costs. I believe that it's possible to develop a "Block I" CEV for the lunar missions that would have a lot of systems commonality with the "Block II" Mars CEV. Primarily, I'm thinking of the avionics (navigation, communications, etc.), electrical power and distribution system, TT&C, and perhaps even life support. The commonality would allow for very good testing of these systems before the Mars missions and a shortened Block II R&D cycle.

I use the JSF program as a model. The F-35A is intended for Air Force use. The F-35B will be for the Navy and will have larger wings and more rugged landing gear to handle carrier operations. The Marines and British will use the F-35C variant (the Air Force might buy a few hundred of them as well) with provisions for STOVL operations. All of these planes have upwards of 80% commonality, especially in regard to the very expensive engines, avionics, weapons and other systems.