I believe it's 50/50 of the above mentioned minerals. You get an idea of the cost look at the cost to add a slipway or to add 1000 tons. It will be half of the cost in Duranium and the other half of the cost in Neutronium.
Yes, there are costs - I have found my dockyard expansions halted due to lack of them. I believe the minerals required are Duranium and Neutronium. I do however not know where to check, or what amounts they cost.