Yeah I agree, but these tests can't show the cost without the base layer of mitigation (since at least PTI should be in effect across all instances at Amazon),

Yeah I agree, but these tests can't show the cost without the base layer of mitigation (since at least PTI should be in effect across all instances at Amazon), so they're incomplete. The "unmitigated" version have at least the host-level mitigations. I'd love to know more definitively whether host-only mitigation is enough.