Thin margins are their own fault
The original Surface was basically their attempt to copy the iPad, and it was a miserable failure. No one wanted tablets from Microsoft. So they did an about face with the brand and turned it into an ultrabook on steroids. While it was still "technically" a tablet, it was sold for and purchased as a laptop in almost all cases.
At first the Surface Pro was a premium line priced in Macbook Pro territory, so margins wouldn't have been an issue. Unfortunately some dim bulb at Microsoft decided they needed to move it downmarket, so now you see low end versions for as little as $799. Given the cost of making it so thin and light, there can't be much if any profit at that price. Sure, they probably get a bit more market share that way, but less overall profit - many low end buyers would have bought a better spec'ed version at twice the price if the low end version didn't exist.
Yet another self inflicted wound.