Yeah, travel insurance with the word "cancer" mentioned anywhere is notoriously expensive. Usually more than the cost of the actual holiday. Even for those who have had the all clear or just went in for further testing, and never had a cancer. They basically assume you'll end up needing some sort of care and they want you to pay for it all. You'll never stumble upon a poor insurance company.

I'm sure this site has an article on insurance and the best places to get it from, but it will be expensive regardless. I don't suggest you do this, but there's people who take the risk and go without.