Yeah, bandwidth is not free. You always pay for extra bandwidth. For smaller scale things (like band sites etc), you're generally limited to n GB/month for bandwidth. Say, my site for Vortech has a 10 GB/month limit, and I have five Vortech releases there for download, 50 MB on average. Lets say, there's 100 album downloads, or 20 (twenty!) people downloading all five releases, that equals 100*50 MB = 5 GB of bandwidth, which is HALF of what I'm allowed each month. Once you reach the bandwidth limit, your site is shut down until the beginning of the next month, which isn't acceptable for a band website.
Now for a bigger service, like Myspace, they're renting bandwidth on a transfer amount basis, ie. instead of set limits for each month, they pay a certain sum for each GB transferred. These are contract-based, but let's assume a sum of 10 cents per GB:
- Say, 100,000 bands, each with 5 songs uploaded (á 5 MB)
- Every band's five songs are listened to 100 times in a month
-> 5 songs * 5 MB * 100 listens * 100,000 bands = 250,000,000 MB of bandwidth usage, or 250,000 GB
250,000 GB * 10 cents = 25,000 USD per month. If the song limit was 10 MB, you'd pay 50,000 USD per month. All the sums above are VERY modest estimates.