I suspected it was more regulatory than economics. As with coal vs natural gas and oil, there is always an economic tipping point.
It's the regulatory framework that makes the economics of nuclear plants so undesirable. You gotta jump through a lot more hoops to build a nuclear plant than a coal or natural gas place. As a result, at this point uranium plants are still not cheaper than coal. These thorium plants would probably cost a lot more than uranium plants -- the advantage is that it destroys weapons-grade plutonium and produces waste that is very difficult to turn into a nuclear weapon. Since it's proliferation concerns and not economics driving the fuel in this plant, I suspect it's heavily subsidized by the Indian government.