Fastmail nets around $50/year (if you pay annually) for mail + your own domain, and if you don't need the domain, it's less.
Another $50/year really isn't that big of a stretch for some people, but I can see how saving up to 70% on email would be a pretty easy choice for those who don't really care that much about email and might even use gmail if it weren't for needing a custom domain.