I got 2 vanity tops cut from a remnant of a slab for $350 each. That included them measuring, cutting the granite and attaching a sink (included in the $350), drilling holes for the faucet, and a backsplash/side splash. Then delivering and installing, of course.

You definitely want to find a remnant you like and work off that, it will save you a ton of money.

So I think you are just looking in the wrong place.
At least around me you would never get that kind of deal from a retail shop, who doesn't stock or fabricate the material themselves. They order the material, mark it up, send it to a fabricator, who they pay and mark up his/her work and then sell it to you. What ever is left from the slab from YOUR job, the fabricator does what he/she wants to with it if you don't ask for it back. THAT is what you want from what OTHER people have done, lol. So you need to go to a local stone fabricator, see what left over pieces they have, and hire them directly to cut it and probably install it for you. They will attach a sink they or you provide for a little extra.

Drove to 4 stone yards (in March) looking for a 5' vanity top from scraps. One had no scraps even near the color needed. Two others had various pieces we could use, both quoted ~$900-$1200 (cut finished and installed w/sink).
Yes, for remnants.

Finally a neighbor suggested a 4th yard where they miraculously had a scrap of the exact stone I wanted, and they did it for $600. Template, cut, finish, install, faucet holes. I considered that a bargain after getting the other prices.

YMMV