In my opinion, there is without doubt a market, but your main challenge is going to be marketing your themes.
In a professional context, paying $29 or $50 or even $up-to-a-couple-of-hundred for a base/starting theme is a no brainer _if_ it's good enough, and you're convinced it'll not be a nightmare to adopt/extend/maintain in the future.
That "convincing" bit is hard...
Like pickpuck points out, ThemeForest will be one of your major competitors, so you'll need to work out good answers to two fairly difficult questions:
1) How are you going to differentiate your themes from free or inexpensive ones from ThemeForest?
2) How are you going to compete on marketing - SEO/Word Of Mouth/developer "mindshare" against ThemeForest (and al the other longstanding well-SEO-ed theme suites in Google)?
Looks to me like the going price for static templates is $20-$50.
https://themeforest.net/category/static-site-generators https://themeforest.net/category/site-templates https://jekyllthemes.io/premium