Floating to the gold base

First of all, the gold base provides an income only 5% greater than a regular base, which is not much at all. The key is that you need much less workers (less money invested, shorter time) to reach an income even higher (if not that much higher) than normally.

The way to examine the problem is to introduce the functions for total minerals mined as a function of time, for both the gold and the blue bases. These functions comprise little linear functions, each 17 gs long, because that is the time required for building an SCV. In such a 17-gs-long interval the mineral collection rate is constant, so the function is linear.

Now the gold function is delayed by the time it takes to fly to the gold base. If this delayed function intersects the blue function before the gold base is depleted, it means you have a window of opportunity, where you actually have an economic advantage, until the gold patches run dry.

For Habitation Station, the map where this strategy was frequently used, the graphs look like this:
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Surprisingly, the time it takes to fly to the gold is 45 gs on Habitation Station (much more than I expected), which results in the strategy only paying off after 6:17. The minerals are depleted around 13:58, when the economic advantage of the floating Terran is 227 minerals, so about 8 Marines, considering he needs 4 less SCVs, too. This may seem a good number of units, but at the 14 minute mark, it is not all that much (although it may decide the game in some cases).
You can decide to attack earlier, though. At 6:17, you will already have a 200 mineral = 4 Marine advantage, which gradually grows.

On Foxtrot Labs, depending on the spawn location, it can last either about 58 game seconds or 1:30 game minutes to fly to the gold base. In the closer spawn, the gold base will first pay off at 10:50, and the base will deplete at 14:11, with an advantage of only 82 minerals. In the longer spawn, this strategy never pays off, with a deficit of 277 minerals when the base depletes. This build does not work on Foxtrot Labs, especially not on longer spawn. Do not try it!

The effect of MULEs may not be taken into account, becase they only start when the CC is upgraded; their extra income is partially compensated by the SCVs taken from minerals due to building; and they are not alway used (sometimes a scan is preferred). These three factors cancel out the effect of MULEs almost entirely, so the calculations should remain valid.