greywar, in GC1 I've managed to have a tax rate of 0% while having 100% funding in effect. Haven't pulled that off since they changed starbases, however.
One thing that helps is to rush for the improved government types early on. Yes, "Interstellar Republic" increases your maximum cost because it increases your ability to spend money, but it increases your income by the same amount. Plus, the fact that the fixed expenses don't change means that they'll be taking a smaller percentage of your overall income. Seriously, I rush that so hard that I'm usually a Star Federation before anyone else makes Star Democracy, and I strongly suspect that the only reason they made "Interstellar Republic" is because I sold them the tech.
Let's take a pretend Imperial empire that is making 1100BC a turn on taxes, can spend up to 1000BC a turn on their manufacturing/research, and spends 100BC a turn on maintenance. So, their budget balances. 1100-1000-100=0BC.
Now, if they go Republic, the 1100BC in taxes turns into 1375BC, the spending of 1000BC goes to 1250BC, and the maintenance stays at 100BC. 1375BC-1250BC-100BC=25BC surplus. So, they're manufacturing more faster AND have a surplus of cash. If they set the finance slider to 80%, they'd get the same 1000BC of manufacturing/research production that they had before, but now instead of breaking even, they've got a 275BC surplus.
My biggest problem with my economy is that I grab the higher PQ planets first, but it's the PQ 6-9 that I use for my tax base. At those sizes, you can get enough of a population to make econ improvements worth it with only a single farm and little to no morale improvements.
So, at some point I have to start diverting colony ships from expansion to filling in the "economy role" planets.
My home planet is usually where I put my econ capitol and dispatch freighters from, because it's the one PQ10+ planet I have that will have a population larger than 3.000M. I NEVER put farms on a PQ10+ planet until I've got enough techs/goods to keep them happy easily. As long as I'm going to have to deal with the morale issues, I might as well make some money off of it.
Oh, and I don't know how many of you know this, but if you expand that way (filling in the smaller planets later) you can usually get away with stripped down colony ships. A default colony ship costs 155BC worth of manufacturing capacity. A colony ship with all the life support and engines ripped out only costs 85BC worth, so you can backfill the planets much faster if you keep designs like that handy. The same applies for constructors. Unless you're afraid that the starbase might get destroyed or you might want to divert the constructor to a newly found resource, there's no reason to have any lifesupport on a constructor that gets sent to upgrade a starbase, since by definition, the starbase will be in range of a ship with only the built in life support.
Oh, and a custom built colony ship with an ion engine instead of a hyperdrive engine not only has more range (1.1 vs 0.9) but is 15BC cheaper in manufacturing capacity, so definitely design that ship as soon as you get that far. 10% more colony ships can help with the rush quite nicely.
Yet another tip on the rush. Don't forget to research Galactic Warfare and Space militarization once you have your economy established, as that will give you +20% to your military manufacturing rate.