There are two thigs that make it costing more than Windows unsurprising.
Firstly the cost of Windows to manufacturers like Dell is much much much lower per unit than the likes of you or I would pay personally, and they get a kick-back for every bit of crapware they install on it for you which could easily make the Windows+crap solution zero cost. The crapware is not available for Linux, so they lose that couple of $/unit.
Secondly, if they have done as much work as "the result of a skunkworks project to optimise the open-source OS to run on Dell projects, to create better laptops for developers" might imlpy, then that sort of work to any decent quality level costs a fair amount in experienced man-time. Most chipset/device manufacturers produce their own Windows drivers that are (eventually, usually after a few revisions) fast and stable, but produce very little or nothing at all for other OSs such as Linux. This means that anything not yet fully supported and optimsed by the mainstream kernel woudl need work from Dell's team - and it may not be easy work as often public documentation for such things is sparce or otherwise lacking (or simply not available: they may have had to pay for access to some information).
This isn't about creaming money of us silly Linux people - it is about not doing work for nothing (which is fine for individuals and small groups who are making use of what would otherwise be spare time, but very difficult to get passed your shareholders when you are a publicly listed company).
As Windows gradually loses market share due to the number of devices (I'm including everything here, not just desktops and laptops where Windows is still very much king) running other options (Android, iOS, Linux, ...) the device support situation will hopefully change to the point where (at very least) good documentation is publicly available for most things.