After you have a sturdy fleet of some kind, and have done the math to see that you are using 1/3 of what the resource box reward gives you, then you have your next way for income.

If you beat i30, it gives you an i30 reward. This treasure box comes with either a blueprint, or metal/he3/gold resource. It is about 1 million resources of whichever kind. If you continually farm i30 over and over again, then thats the same as adding 300k income per hour to your gold, he3, and metal income.

Some instances are easier to farm than others, in a time sense.

An Example is i13, you have an option to place more fleets in i13 than the pirates have set up. Doing these kinds of things will open a window for you to get a continuous BluePrint income, to sell or trade for MP or Gold.