I guess it depends on city and state.
Where I am, I've seen prices from $500 to $1000 for interview part. Maybe there are lower prices, I did not shop around that much.
The attorney has a prep phone call, explaining how interview is conducted. They know location well, so they explain in detail how to get there, even which door to open and which room to go
You are absolutely right about attorney not asnwering questions for client at interview, however during prep call you can ask any questions about how to answer things. Also, attorney can take notes about questions asked and answers their client gave during interview for further record. They can also help keeping interview focused, and reduce chance of adjudicator asking irrelevant questions. Attorney can help getting USCIS supervisor involved too if necessary. If decision cannot be made on day of interview, at least attorney has full picture how it went and can help with next steps (answering RFE, suing for decision etc)