Residential vs commercial are very different. For residential in most situations you can provide a customer with a small base line of storage for their cameras and increase with extra fees (20GB would be a lot for most residential applications).
For commercial assuming 1080P at 15FPS (our recommended settings) you can assume a camera recording on motion (50% recording) will use about 320GB/month. So 1 week is about 80GB/camera/week (recording on motion).