DEFINITELY worth the cost.
My, usual, invasion force, once I hit tanks.... 2 transports, 7 tanks, 1 " defensive unit." The defensive unit is anything that has a few city defense promotions on it, i.e. Marines, Infantry, Machine guns, etc.
Once you have that in place...Go kick some booty. But...Don't forget to keep reloading. Once I start a war, I go into war production mode. I pick my cities to build transports and have them build about 5 to 6. Have a few cities build tanks (You will lose some), and a couple build your defensive units.