If you want to get easy money, equip your battleships with large calibre chain guns insted of artillery. This can easily take out enemies. Replay the mission where you destroy the flying superweapon, and you can destroy it in one pass. Each mission will get you at least $40,000. And up to $70,000.

Destroy 100 Superweapons to get the Gravity Shield. This takes away 85% of damage from all enemy fire. You will find it extremely helpful.