To answer your question, at least one person thinks the idea is a good one--the plumber, who stands to make a killing. As suggested by others, get a few more quotes. Maybe there are some people out there who aren't quite as greedy.
I installed my own sump pit when the original one (about 20' away) couldn't keep up with the inflow--took me just a half-day, and about $136 worth of materials (and that included a new pit, gravel, pump and pipe to the exterior). If I was to do it for a living, I could get very rich by charging just $1000 or so.