Having a custom page shown (or redirect) when the origin server is down is a very nice feature even for small sites since users may not know where to look for updates on site status if site itself is down.
Would it be possible to offer this feature as a paid option similar to buying page rules or workers that is cheaper than the Pro Plan (many small sites may not need all Pro features)?
$20 / month is pretty cheap if you need that feature, but for a very small startup you could also get away with some DIY using Cloudflare Workers for $5/month.
I agree on that.
If you really wanna be prepared for something that critical as a server-failure then spending $20/month is really not much. Also you can (while your server is online) define your own 4xx Errorpages for your sites which are working even on free plans. Just 5xx-Error pages are dedicated to customers (paying users).