But the mining of NEOs could be as little as *zero* dollars per gram (excluding capital costs and maintenance), insomuch as it would be possible to fire sintered minerals (using solar power) via a coilgun onto an aerocapture trajectory.
You forgot to exclude operational expenses. And also didn't mention that you can't just lob chunks of metal straight to Earth's surface, and refined minerals in LEO aren't useful to anybody right now or for the foreseeable future. If they are going to re-enter you need at least some sort of return vehicle to control reentry, if they aren't then you need on-orbit factories for those to do any good. By your same logic, the mining of minerals on Earth would be zero dollars per gram if the equipment was solar powered and automated - that isn't the case, because fuel and drivers are a tiny fraction of the total cost of a mine.
There's no question that it will be economical to mine asteroids at some point, but the primary driver of this is launch costs. Planetary Resources exists because they believe the business case will close, and they are giving it a good shot, but given that we have only one small startup pursuing things right now, I suspect that it's still not obvious that the numbers will work out. Part of that is because there is so much uncertainty about the makeup of asteroids, how challenging the material will be to retrieve and what the real mineral densities are. But I guarantee you, if the cost of space access came down by an order of magnitude, as SpaceX is targeting, a lot of marginal business prospects become viable. That said, Planetary Resources might already be banking on that cost reduction, it's hard to say.
It does nobody any good to pretend that the lack of a space economy is because investors are cowards and morons that are just too narrow-minded to see the possibilities of space resources. The real problem is that boondoggles like SLS and the Shuttle before it cost the better part of a billion dollars for each and every launch, and that's been the primary model for space access historically. If people appreciate just how expensive this is, they'll stop advocating what amounts to a private jet to retrieve a $15 sweater, and instead start focusing on finding better transport. Things won't change until access gets cheap.