If you book your insurance 3rd party, many don't require payment until prior to your pay in full date for the cruise. So, you could also wait on insurance. You just have to read the policy disclosures as some policies have disclaimers around having purchased insurance within 15 days of first payment toward the cruise versus prior to payment in full. We just select a policy that allows for coverage and payment prior to final payment.
Though, I'm sure if you want to buy it now, you could do so via estimating the cost of airfare. If you were to need to file a claim you'd have to show evidence of actual cost anyway. If you're underinsured you won't get the total value and if you're overinsured you'll only get the actual value.