Natural gas is not being switched out for coal. At least not in the USA. I think that might be a typo. I unfortunately can not site a fancy link like you have but from experience I can tell you that the actual costs I have encountered for projects showed that the price of scrubber tech/equipment (and installation) was less then the cost of a plant conversion from coal to natural gas.
The costs of coal are higher than they were before and are growing. This is a direct result of government regulation. The other problem is that these regulations and the continual call for more is causing a great deal of market uncertainty, which is also driving up costs (not just for the coal but for equipment). Natural gas is currently not in the same position, it is cheaper and less regulated (no mining regulations/legislation). Further, because there has been so much drilled for there is a surplus (supply and demand ==> gas costs are lower). This is mostly because of the lack of good, viable, stable export terminals. So when some terminals open in the future we will see higher natural gas prices.