We put in UFH and then an ivory Italian travertine on top of it. The tiles are about 12mm thick and were laid on BAL Flexible adhesive by a professional tiler etc. The floor was poured about 1.5 years before we tiled and had been heated for 6months, so any settlement cracks had already arisen and had been monitored for movement. Despite this, the tiles have been getting miniature hairline cracks in them. (Individuially, as opposed to one big crack though many tiles.)Perhaps the tiles were too large (about 90cmX45cm) or the stone was too soft?...The tiles were also sealed with 2 coats of sealant, so maintenance would be easy. I do love the floor, and would probably make the same choices again, but my heart did sink when I saw the first tiny crack. I am probably the only one who notices them, as I am the one who cleans the floor (!) - the cracks blend in with the veins in the stone and are only noticeable if you look for them. Nevertheless, after all the expense and effort, it was disappointing to see any imperfections. I have no advice on where to source natural stone tiles "cheaply" but I do know of a new crowd in KK who do good prices compared to what we paid when we bought them in Tipp and "imported" them to KK.
We have 3 small kids and the tiles are wearing well . Toddler's skull has held up ok too (frequent bruises from floor), though I shudder to think what the Dr.s would say if he ever needed a skull xray. Just a point to consider.
Sorry this hasn't been much help, but wanted to let you know our experience so you know what to ask your tileshop/tiler.
Sheesh