Why are you asking the going rate? It's a matter of your cost for materials your operating cost and desired profit margin.What is the going rate for a copper connection? With the cost of copper it is hard to keep the cost down, and everyone happy.
But most people can't get away with using a $25 dual check for their backflow.A good plumber can do a basic plumb job in less then an hour and parts for this come in well under a $100, The backflow is on top of this figure.
Most plumbers and guys that claim to be plumbers think they are doctors.
There is nothing wrong with asking what other guys are charging out there, only good idea's can come from it.
No question is a stupid one in my opinion!