Windows does NOT increase the price significantly.
If you compare apples-to-apples (similar CPU, drive space) there's not much cost difference. Some of the Chromebooks can be cheaper by having only a small amount of onboard FLASH (say 32GB) and no other drive at all which makes installing Windows and updating it impossible even if the hardware supported it otherwise.
Lack of System RAM is another serious issue as I consider 2GB the bare minimum especially if you open up several browser tabs.
Windows is even given to manufacturers for FREE on the cheaper mobile devices ($200 and below?), but I believe it only costs at most $35 to the manufacturer for most laptops.
It would cost a lot MORE to buy a laptop without Windows then buy your own copy. For example, let's say a particular laptop was $400 with Linux and a similar laptop was $440 with Windows 10. If you bought Windows 10 for $120 (possibly cheaper) you're looking at $520.
*I strongly recommend looking for a Windows 10 laptop in your price range that has enough customer feedback to determine roughly if the quality is okay.
**Also note that Windows 10 currently has a few issues such as freezing for some people. If you decide to go with a Windows 8.1 system that's okay too, but then you may have some issues upgrading some drivers. I've upgraded a few and mainly had issues with IR drivers (remote), card readers and a few pieces of software.
While upgrading to NVidia drivers went okay on the new desktop and old laptop (9600M GT), lots of people wouldn't know how to even do that much as the upgrade from W7 to W8 in that case was really sluggish due to basic Microsoft drivers being installed (I recommend the Geforce Experience utility in that case).
***Finally, some laptops switch between an iGPU and dedicated GPU so you need to have that setup correctly (such as NVidia Optimus) though a budget laptop should use an AMD APU or an Intel CPU (with iGPU) only.
(Get drivers for video from NVidia, AMD, or Intel as appropriate. Intel for example has a tool you can install to scan for updates.)