The amount of work rebuilding these carb quartets is very high. And a lot of it is knowledge based work.
If all new parts are installed (all jets, floats, valves, gaskets), then the old ones don't have to be laboriously cleaned. All this considered, about 300$ plus parts would be my price to refurbish the carbs. And parts might approach 200$. Postage, packaging and delays for receiving parts can wipe out half a riding season.
So overall, you self-train, self-educate, and self-execute to put these carbs right, or pay pretty big bucks to an experienced expert. The good news is that 100$ ain't much money any more.