Cost of servers & bandwidth 
ashleyw: S3 based on 100k members uploading on average 5x5MB audio files each, and a growth (new uploads) of around 10%/month:  Storage: 1500GB
  Transfer in: 150GB
  Transfer out: 30000GB
  PUT requests: 50000
  GET requests: 10000000
  ———————————————
  Total cost: $4,560.10

Not too bad if you ask me! (btw its late and I'm tired, so sorry if my figures are WAY off! :P)