If the electrical breaker box is with 100 feet and only 120-volt 20-amp wiring was required, I would estimate $75 for materials and $75 for labor. I not much of a plumber, but they don't do anything for less than $100, so I would say another $150 for the plumbing. Are you sure there wasn't any gas piping required? Most dryers require 240-volts unless they are gas. If you had any gas line installations, I would figure another $200.
I think between $300 and $500 is realistic given that the connections were near and everything went into the floor and run exposed in the basement. Find out if there were any unknown surprises that the installer found that may have increased his cost. If he did adjust his price, find out why he didn't tell you first.
17 years in electrical engineering