I think Upload is the most expensive bandwidth..
Charbax, I said 'upstream', not 'upload' this may not be an area you are all that familiar with, especially in the case of UK ISPs and ADSL.
Upload bandwidth is never an issue as far as cost goes because there is a huge difference in the raw ability of an ADSL user to upload compared to download and if they ever saturate what little they have their download suffers badly.
The cost problem is very much focused on the amount of data that moves from the ISP to the customer as the cost of data moving in that direction is many many times higher than any upstream traffic from or to the ISP from the rest of the net.
This is why caching doesn't help the ISP much at all as it only saves on upstream costs.
I have worked in the ISP industry for some years now so this is stuff i am extremely familiar with.