Ships cost because ground unit cant hardly hurt them. it's more realistic since you can shoot down a starfighter with a tiny gun. you get money for every kill. the slayers are very hard to kill. you should shoot them from the air. to get to the supreme overlord, you have to go inot the fortress. ships cant get in there since they're too big and will die.
Your comp ally's base is pretty well defended so you won't have to protect it that much. the comps can build stuff

. you can't just rush in with lots of units since th defences will kill them all. best way to get rid of defenses your units cant kill is with orbital bombardment (nuke). i finally got heals to work

note: there's an outpost outside the base to heal. when you get close to the supreme overlord, a group of your ally will help with the attack.