The F is more expensive than the E, due to it's increase in cockpit equipment and the extra ejection seat, and it is also carries 700 pounds of fuel less than the E, or about 5% less internal. It is my understanding that the Navy purchased enough F's to cover the FAC(A) role as per their SOP (USMC/USAF both do single seat FAC(A)), then used the slightly cheaper E's for all the other squadrons.
As for using G's instead of F's remember, the Growler has a few more differences than just missing a gun. It's got a few minor aerodynamic changes on the wing, as well as mounting the AN/ALQ-218 receivers on the wingtips. This means that without tip AIM-9s or a gun, the Growler usually carries no short-range dogfight weapons. The Growler also has different mission systems than the standard Rhino. Growler front seaters do not train with other (Super)Hornet pilots, but do all training on the jet at Whidbey. The mission equipment also tacks on about an extra $2 million to the price tag for the G, in addition to requiring the extra training.