I don"t know how much is the difference in cost to the ARMv6 (ARM11) which you are using, but probably marginal.
It not "marginal", its not even "expensive", its simply impossibly!
The CPU isn"t a separate chip, its simply part of the SoC (system on a chip) that the RPF have chosen to base the R-pi on. This was an existing chip developed by Broadcom for the mobile device market, its actually more a GPU that has a CPU attached to it as an "afterthought" than it is a CPU which happens to be integrated with a great GPU, but that aside.
The real point is that this is an existing chip, which happens to have an internal ARM11 CPU inside, the CPU alone cannot be exchanged with another one without a complete redesign of the SoC, and this won"t happen unless there is demand from the mobile market for such a chip, which at this point in time isn"t really likely and would also result in a necessary redesign of the R-pi, and a fractioning of the user base in "older R-pi" and "new R-pi" users which is contrary to the goals of the RPF.
But wait a bit and "clones" of the R-pi are bound to happen, not adhering to the goals of the RPF, and obviously these are going to compete on technical grounds, so one with a faster CPU is bound to arrive sooner or later. The "ghost is out the bottle", now that producers are realizing that you can indeed build a "desktop PC" for much less than when using the older paradigm of using conventional PC hardware.
Hopefully the R-pi will remain as is and will not be changed because of this, and will keep a large and enthusiastic following.