[The numbers are distorted of course as they do not take into account that some of the 'profit' has to be used to pay back the R&D that went into the devices.]
There is not teardown cost analysis work. None of the other teardown ever has to account for so called R&D cost, so it should not be any different for Surface.
The profit margin is distorted not because of the R&D, but the Windows tax itself. Sure, Microsoft does not have to pay for RT and Office licensing fee, but if you view the hardware division is merely another hardware competitor in the market, then they too have to include the $100 cost in the finished product. Hence the RT is more like $370 in cost, instead of $270. There is not a lot of margin to be made there really (the remaining profit has to be distributed over fixed cost such as R&D marketing etc), compared to Apple or more premium Android model. Similarly, that's more or less the profit margin you will be looking at for other RT models as well.