As I earlier said long time back, their should be a fixed charge that a customer have to give for maintaining his stb active whether he watches 1 channel or 100 channels (paid or free). There should be a criteria something like : NCF is, say, 150, then till 150, total cost of channels that a customer choose he have to give only 150. Just like our electricity bills, there is a fixed charge.
For ex, if a customer buy channels worth rs. 150 he have to give only 150.
If the customer buy channels worth even rs. 1 above 150, he have to give next additional charge with 150.
Just make it simple & easy for customers.