Ching > if only drink beer(heavy drinker like me haahaa...), I think around 400 is reasonable (include DJ tips & free snacks)
Most KTV in CP charge 468 per dozen (buy one get one free) = about 19 per bottle
20 bottle = 380 (also Manager will free at least 1 - 2 dozen)
I drink Chivas , beer non- stop from 7pm till 3am is around 500 also.
I think you should lower yr charge. (Just my POV)
Hope you can make it & Have fun !