"The cost of retrofitting in an already existing floor is very high."
It depends what kind of system you're having.
We had underfloor heating installed in our kitchen and it's LUSH - I love it! But we had the electric kind which is literally just wires which go down along with the adhesive. It's no different to laying an ordinary tiled floor and you can have it in as much or as little of the house as you want (although only if the floor can take it, obviously. It's not suitable for all flooring).
The water kind is cheaper to run but much more expensive to install.
In our case we have it as a top-up to the existing heating (we still have a radiator in the kitchen) - it's mainly to take the chill off the slate floor and make it comfortable to walk on. Using it to heat the whole kitchen would be more expensive, and we didn't want to remove the radiator so it wasn't a problem for us.
If part of it ever fails then we've already decided we'll just live without that section - it's not easily accessible, you would have to rip up the whole floor to access the wire, so the options would be basically new floor and new heating system, or turn up the radiator and cope. Having said that, I believe it comes with a lifetime guarantee - not sure what that means in effect, but I guess we'll find out if it ever goes wrong!
If you are planning on using it as your sole heating system then I would do a lot of research and be prepared to pay quite a bit.
If you just want a nice warm floor so you don't have to wear slippers, then a cheapo electric version is very easy to install and just the job.