That greatly depends on the KTV and what you want to have happen inside the KTV.

Normally they charge a flat fee for the room or for a period of time.  That fee can range from 200-5000+ depending on the KTV.  If you want drinks, food or companionship, that will cost extra.

The prices will range widley depending where you are and what kind of KTV you go to.

For normal KTV they will charge you for the room by the hour based on the day of week, time of night and size of room...anywhere from about 30rmb an hour for a small room early in the night to 160rmb  for a giant room during peak hours.  Drinks and snacks will vary in price depending on location.
A typical night at KTV in Shenzhen with my gf and 2 other friends on a Saturday night will run me about 200rmb for the room and about 400rmb for drinks and snacks...600rmb for the night with enough booze to get everyone feeling pretty darn good and about 4 or 5 hours of drinking and singing.  Of course on off days like Sunday you can often get a package deal like room for 5 hours and 3 dozen beers (or 3 bottles of wine) and a few snacks for around 400rmb.

Now if you are looking for a "hostess" KTV experience, then this is a different story.  Room rental will be between 200-500rmb an hour, plus drinks (expensive).  You will also be paying "tips" of about 150-300rmb for the night per girl that you pick to pour your drinks and keep you company for the evening as you drink.
There are many different levels for these types of KTV with the priciest being in high-end giant hotels.
Of course some of these girls will try to get you to hire them for "additional" services after KTV for large sums of money so be aware.
Happy Singing!