Best way to make fast money, increase everything BUT monster health. The other stats which increase the money gain do so at higher rates than the health upgrades. Also, keeping health low but spawn rate high means faster kills/faster rate of income. When damage is upgraded enough feel free to increase health so long as it doesn't surpass a 2-hit kill for maximum income.
Build 10 strafers, 10 bombers, and 4 guppies. Have good defensive position. Power supplies and ore as needed. Bomb/strafe one are next to target until a good clean area is made of Anti-creep. Hot-load both guppies to there with packets and build a nullifier. Won battles where half the map was covered in creeps that way.