Greetings
There is a standard ... like one hour = 60 minutes is a standard.
If the people are THX or ISF, they will be adjusting the TV to that standard ... or they'd better be ...
There is no set amount that people charge ... there is a range and most fall into that range. The market will ultimately determine what the price point is.
Charging $300 may be a big deal in one part of the country, and too little in another part of the country.
Regards