How can I calculate the effects a floor coverings thermal conductivity will have on my heating costs?
Respectfully, you are trying to compare apples to autos here.

Burning gas makes things hot. 

Thermal conductivity is just how fast it takes the heat to get to you. 

I suggest you go try to calculate it from theory, physics is fun, calculus is interesting, the numbers will not lie to you, and you will see the logical error immediately.

The heat has to go somewhere. A metal floor would get hot fast, and cold fast.

A concrete floor will get hot slowly, and get cold slowly.

If your floor has a very low thermal resistance - you would be able to feel the path of the heating pipes/elements. It would have hot spots and cold spots.

With a high thermal resistance the entire floor will feel the same temperature.