You have to factor in import taxes, exports costs, and most importantly branding - both Roewe and MG are advertising that they are 'British' cars and not Chinese. Also, material resources for car production are pretty much the same price all over the world, especially if you're going for quality stuff.
Americans would lap up something that is 'British' - refined, luxurious, elegant etc - in short, the British stereotype is easier to sell than the Chinese stereotype.
Plus Im sure MG are getting huge tax incentives for investing in OK, bringing USD investment back to America and setting a trend for other Chinese companies investing in USA, rather than the other way around.
Its a good thing for both parties.