This has been debated at length before; and Drake is right, the decrease is 1% per turn, no matter how many (or how few) miners/farmers/extractors are being used per turn. So that starbase could mine a 144% asteroid field for 144 turns.
Well, OK, slightly less than that, since eventually maintenance will be higher than the resources mined; but with Mineral Miner III's, a Master Computer (slightly less expensive than all those life support and crew quarters for a starbase), and the 50% maint. reduction for bases, that comes to, um, 5000 minerals/125 orgs/725 rads maintenance cost; 24 miners extracting (800*value) each means that you make profit on any asteroid (or planet) with mineral value higher than 26%. So, in reality, that Starbase loaded with mineral miners could mine for (144 - 26) or 118 turns. Obviously, you'd want to scrap it at that time. Which would return a little value also...
