Except it's not a pure cost thing. Delivering blockchain/ICO emails impacts the deliverability scores of everyone else routed through those same domains and IPs. It's incredibly difficult to pin that to a number, because it's hard to quantify in advance how much of an impact one customer's blockchain emails will have on your IP/domain reputation.

The only way to make it a pure n% cost is to isolate the individual customer or all blockchain-email customers to a dedicated set of IPs/domains that can have their own (probably: really shitty) reputations. At that point, it's highly likely you'll go through all that effort just to take money from people and still not be able to deliver their email due to catastrophically low reputation infrastructure.

Refusing to accept the impact to your other customers of a hot spam topic is not "censorship". People really need to stop misusing this word. Mailchimp is a business, and has every right to take reasonable steps to protect their customers. It's already pretty common for porn and get-rich-quick businesses to be refused by these services, largely for the exact same reasons.