Does ISP's pay according to Data usage?

[paid]

I have a query that whether ISP's have to pay someone according to Data usage or they limit subscriber usage to protect their network congestion. :roll:

[freshseasons]

As per my friend who runs a Cable internet Service in this city ...he works this way ...
He's got a 2 MB leased line from VSNL HTTP. Now he does this ..
He distributes the lines in this way ..40 Kbs for people with normal usage , 64 kbs for devoted , and 128 Kbs for cafe' usage and at most 256 is some one speacial wants for say to manage server loads ( if the subscriber is running server )
And this means to him is ....theres no restriction on the Mb Data transfer . All he has to pay for is 2 Mb Line per Sec and use it to distrubute in the area.
So even in the night when there is no Data transfer he still has to pay as per the time. The payment is in terms of Per months to the Gateway Holder wheater the line is used or not.