I'm researching the cost of serving old blocks to new nodes that need to synchronize and download the whole blockchain. I can indirectly estimate the bandwidth consumed to serve old blocks, if I know how many new nodes join/leave the network per month. For simplicity, I will can assume that each of the new nodes needs to download the full blockchain, and each node that leaves will not come back.
The more direct way is to instrument a geth node to measure the bandwidth dedicated to serving old block requests, but nobody seems to have done it while publishing this information.