Hmm. I mean, that's a more complicated question, and it really depends. If you want to use automines instead of asteroid harvesters then you'll need a bigger fleet of freighters to move things around. And you can't give them automatic orders, or at least not as easily. You could put out contracts for your civ freighters to move things, too. But ultimately it's probably easier to just park a fleet of a bunch of asteroid miners out there and only bother to move the mass driver. But again, that's not an efficiency answer, that's an "I'm a lazy sack" answer.
If the question is really when is the break-even point in minerals harvested if you build a miner ship instead, then you'd have to factor in freighter transport costs with automines, relative density and accessibility of minerals. That gets into the sort of questions that actually make up the gameplay.
Unless someone has actually done the breakdown, with which case, gimme yer spreadsheets!
