Interesting that your experience was a double increase in costs from going up in security level. My experience was it was an order of magnitude (10x) at each level .

E.g., OFFICIAL let's say is $1,000,000 PRIVATE becomes $10,000,000 SECRET becomes $100,000,000

What we did experience was the cost in time to deliver a service doubled however. E.g., OFFICIAL was 3 years, PRIVATE was 6 years and so on. I expect it varies significantly between types of system, ours was a SaaS platform that read telemetry from embedded systems on assets in the field, so I'd assume that would be one of the drivers for my differing experience than yours.