In terms of man hour costs to Dell to verify that everything is working, I'd guess that the Linux laptop costs more for Dell to produce than Windows.
This is the same reason why other sellers will happily give you a computer without an OS and take the price off, but they won't give you any even basic support for installing anything else.
If Linux users were willing to pay more, then there'd be an incentive for sellers to put the effort in pre-install Linux.
Also, that final OS support cost per laptop is total_dev_hour_to_support / number_of_machines_sold. So if the denominator is small, that can still be a big enough number to surpass $OEM_Windows_cost.
We (sadly) don't get the luxury of pretending the markets for Linux on laptop and Windows on laptop are identical except for the OS.
This is the same reason why other sellers will happily give you a computer without an OS and take the price off, but they won't give you any even basic support for installing anything else.
If Linux users were willing to pay more, then there'd be an incentive for sellers to put the effort in pre-install Linux.