Any recommendation for an inexpensive headless CMS?
I find Contentful too expensive.
I'm using Ghost, hooked up to my Gatsby blog. https://ghost.org/
My buddy is working on one that provides an API on top of Google Drive and can export a Doc to Markdown, https://droplo.io/
Ghost is solid, especially if you want the front-end written for you. I wrote a post recently about how to hook it up to a $5 Digital Ocean Droplet: https://blog.stephsmith.io/setting-up-blog-with-ghost-and-digital-ocean-droplet/. With Ghost you can design your own themes as well, however I haven't tried that.
If you want a truly headless CMS, I tried Butter (https://buttercms.com). They have good docs and really easy to set up (or at least it was with Node).