Inspired by day9's latest cast, the noobie tuesday, I've made a little doodad to help you calculate how many unit-producing structures you can afford on any number of bases. You fill out which buildings you want to use, what mixture of units they should produce, and the doodad will tell you how much percent of your income is required to sustain it.
In day9's cast, he looks at a replay of a gold-level player that builds 3 warpgates and 2 stargates off 1 base, and they are mostly idle, since he doesn't have the income to use them.
The Protoss unit cost doodad is at:
I'll add a Terran version probably tomorrow. The Zerg version requires a little more thinking, but shouldn't be too long after that.
Update: Terran version available at:
Update 2: Zerg version available at: