It has an insurable value which is the value that you place on it, though if you say that a $10 poster is worth $1000 and it's lost, even if it is insured for $1000 they could challenge the valuation. I always insure for what I think the fair market value of the poster is, or for the purchase price of the poster.
Regarding customs, I don't know how that works. Presumably, there is a way to avoid paying any VAT or other tax if it crosses borders, by disclosing that it is yours. But, someone else with more knowledge will have to answer that one.