That math may work with subsidized phone prices, but most Americans (which is where the surface is launching) think of a phone as $200 device. No combo pack priced over $1000 would move enough units to help windows phone market share. Now if they did RT bundle that came with a Windows phone 8 device and only priced $100 to $150 above the rt alone, that could have a real impact.
Or they could replace their buy a pc get an xbox promo they run sometimes with buy a Pc get a free windows phone. That would really help market share.