they make a lot of assumptions. what you need to know before you ring is: what are you trying to achieve? if you don't know, then a professional production house will help you (but be warned that the majority will not be able to). in fact, it's best to actually meet with a video production company first to discuss what you are trying to achieve and then they can come back to you with a solution and cost. otherwise, for them to try and work out a price they need to know how long do you want the video to be and how many days of filming for a basic 