Is radiant heat cheaper than gas heat?

If you are talking about operating costs, it would depend on the prices of the energy sources and the efficiencies of the heaters in question. You must compare the efficiency ratings of the two heaters in question and then calculate the cost of using them in your particular application.

As far as unit purchase costs and installation costs, it depends on what units you are considering for installation.

Right now, in North America, at least, and likely elsewhere as well, the cost of heating with fossil fuels is becoming more and more expensive than heating with electricity.