knowing the way games workshop is doing things currently we'll probably see the following for BFG:
plastic kits for each size of ship that will have the parts to make any of the classes of the given size, so for example - Imperial battleship, Imperial grand cruiser, Imperial cruiser, imperial light cruiser, Imperial frigates
Frigates will probably be the only box to contain more than one ship
cheapst set will probably cost £25, a mortgage, your first born child and a suitable offering to cthulhu