For a site the size of ProtonMail, $6K is the cost for protection for a single month. Most of the companies that offer this kind of protection require you to sign a one to three year contract.

There are two kinds of protection, basic HTTP/HTTPS and DNS only (done with DNS and CDN like servers co-located at peering points), and traffic filtering that is done through BGP with and a GRE tunnel. While you can get basic HTTP/HTTPS and DNS from CloudFlare for $200/month on a business account, what ProtonMail needed was a BGP/GRE which at it's lowest price is a multiple and an order of magnitude more expensive.

Isn't Cloudfare around $2,000 a month with no data caps for high-end package with $50 a month for low end? I know reasons why some people avoid them but I figure there's a similar service in Switzerland that just costs a bit more. That might be what they're referring to for $100,000. I'm curious.