Travertine is still in vogue (though I have heard that designers are beginning to shy away from it.) It is harder to install because it has square cut edges typically. Unless it is the travertine with chiseled edges.

In order to look correct installed, it should be installed level. Tile often has cushioned edges that allow a certain amount of latitude as it conforms to the unevenness of a sub-floor.

And the flat bottom of travertine requires more care in the installation whereas a tile has a stamped bottom or ridges, etc. that can offset the combing of the adhesive somewhat. Plus travertine, like other stones, must be cut on a wet-saw where many tiles can be cut on a cutting board. And it is softer so sometimes will break even in the handling or cutting. Adding to more time and care.

So the extra demands of the product are what causes the installation prices for it to be greater than a conventional tile, as one would expect. In addition, inside the home travertine needs to be sealed and re-sealed to maintain its appearance. It is a product not usually purchased by the poor or frugal.