We base it on how much it cost to ship our past box game projects, such as Zombicide and Rivet Wars.
In the USA, this works out to about $11 per package when shipped via Fedex Home
In Europe, costs increase the further you go out of Germany. The base cost is around 5 Euros for import and logistics fees, with actual shipping in Germany at under 5 Euros (so 10 Euros total), France, and UK at 12 Euros (17 Euros total), Italy and Spain at 17 Euros (23 Euros total). This is for shipment via the very reliable courier service rather than national post. We'll accept shipping payment for European backers in Euros instead of US$ so as to avoid double conversion fees (your Euros to our US$ to our shipper's Euros... madness!)
In the Asia Pacific region, the base cost is $3, and shipment increases the further out from Shenzhen it is to ship. Hong Kong will be under $10 and Singapore under $20. This is via Fedex international - Australia is particularly expensive for Fedex but we're investigating China Post if you are willing to take the risk of the public post losing your item.
Closer to the ship date, once we have all the weights and box dimensions of unlocked add ons we'll be able to give you all a very clear idea of how much shipping will cost, but we don't expect it to deviate very far from this unless there's a major change in courier rates.