We've used the same attorney on and off over the years.
For the N400, my wife filled out the forms. The attorney is charging $600 to familiarize herself with the submitted N400 (1 hour), a 1-hour strategy/prep session before the interview, and the interview itself. So basically $200/hr for 3 hours of her time.
We definitely want a witness there in case the USCIS officer acts of line (rare, but happens, and much less likely to happen with an attorney present), if we ever get into a "USCIS's word vs. our word" type of situation, etc. Plus the attorney knows pretty much all the officers there and knows how to handle the few oddballs. And if we run into the rare problem post-interview, like a denial, we already have an attorney that's familiar with the case and can get working on next steps ASAP.