When you evaluate cost effectiveness you have to look beyond unit cost to what capabilities and effects the taxpayers actually get for their money. The F/A-18E/F is pretty good at short range strike missions, but it has less range and requires more tanker support for longer range missions. It's also generally inferior for the fleet air defense mission, especially compared to a notional "Super Tomcat 21" equipped with a modern AESA radar. Fortunately the US Navy hasn't had to actually defend a fleet from air attack since the Vietnam War, so the Super Hornet deficiencies haven't really bitten them (yet).