Any insurance for the shipping to CGC is on you. After arrival, the book is insured while at CGC and for the return shipping. So, IMO, you should insure based on the estimated grade in the slab. As mentioned by others, don't over estimate. An over estimate will result in an over payment. With the exception of a Walk through tier, CGC will not adjust the the value down, only up. There is no broad based analyitic answer or even a Rule of Thumb to give you. Put what you are comfortable with. As you start to see the grades returned from your submissions you will get a better feel for what to estimate.