Really depends on the postal service you use, and what country your in, usually the cost of postage is worked out either by the size of the package or the weight. Some couriers may charge a premium for live animals or plants as well as fast delivery.
Best packing for fish would be if you ca get them breathable bags, failing that normal fish bags from the LFS/ pet shop and double bagged incase the first bag springs a leak. Then pack newspaper or other suitable material around the bag to keep it stablised inside the postal box. Depending on the weather and the species of fish you may need to add some sort of heat pack to ensure the fish don't freeze in transit.