You can actually get under $100 if you compromise the specs just a little.
There don't seem to be any builds anywhere for the 4.1 (is it an uncommon phone?)
For argument's sake, let's say you bought the 6.1, now your option is to download an "unofficial" ROM from some pseudo-anonymous person with an anime avatar from XDA, hosted on whatever free file hosting they can find today.
Don't get me wrong, I used Lineage when it was called Cyanogenmod, and it saved one phone from landfill which is great. But I'm older now, and I need my phone to work and work well - I really don't have time for problems like https://forum.xda-developers.com/showpost.php?s=76595163ac85.... For some people, that's worth the money.
Edit: here's all the proprietary blobs that'd need to be reversed to be audited to make sure they're not selling you out, too - https://github.com/nokia-dev/proprietary_vendor_nokia/tree/a.... Props to HMD for publishing them, though.
I'm hoping to find an alternative for friends and family who still have the 5X - it's been dropped from Google's security updates for a while, because their support ends abysmally quickly.
More on topic, the LineageOS support requires a volunteer maintaining a buildable tree for the phone, and testing will require the hardware itself. I'm not sure how much CI/CD helps here.
The Nexus 5X supposedly has a maintainer with LineageOS, but maybe they're busy or no longer own the phone.
In my opinion their progress has been fantastic, and I am quite excited for the future of Linux smartphones.