Wow! That cheap? That's usually the cost of the stone alone, with no fabrication. Be sure you can handle the reality of marble instead of the fantasy.

Marble patina · More Info

Seams are often necessary with tub surrounds, due to the fragility of marble especially when working with narrow pieces of stone. Often times, these can be placed in an inconspicuous place. A good fabricator/installer can line up seams well so that they are nearly invisible. Of course, this also depends on the veining of the marble. Ask your fabricator where the seams would go, and about how they would match the veining.

Likely, the price difference is due to the large footprint, and extra material, it would take to make from a single piece of stone. Also, the extra care and manpower needed to handle such a large, fragile piece without breakage.