Sounds like a flat rate manual.
As far as material, you have 2 angle stops..$10 each, 2 sink strainers...$15 to $50 each, water supply lines, assorted tubular or schedule parts for the continuos waste and trap. Miscellaneous solder and or glue, gaskets, etc.
$300 sounds a little high for just material.....but realize that any plumber has to mark up his stock material, and he probably buys professional quality items, and pays more for it than a price you would see on the shelf at the home store. Still, $300 is a little high, but if they flat rate the job, then it is what it is.