SBK, I haven't actually purchased any fleet ships but I think they have a cost that = #fleet credits + #ship modules. The modules are available from the store and exchange. If you are a subscriber, I think the monthly stipend is enough to pay for one module.
Fleet ships have an extra power that store bought ones don't. Take a look at the ships available at the fleet starbase.
Update - Thanks Jim Gamma \/ for the clarification.