Some online services are pretty obvious as to what the costs involved are. Domain registration, for example, is a flat yearly fee. You'll hardly ever see it portrayed as anything else. Streaming media hosting, on the other hand, has several factors that influence what you pay, and while different companies might have different pricing schemes, it all comes down to a simple equation:
Bit Rate Compression x Length of Media Clip x Number of Times Media is Accessed = Cost
Bit Rate Compression
Bit rate is the number of bits that are transmitted in a given time period, usually a second. Because one bit is the smallest unit of data in a computer, thousands can be transferred per second, which is why modems and other telecommunications equipment are sometimes gauged in Kbps. A 56K modem can transfer 56,000 bits per second (assuming perfect conditions exist on the network). When you encode your media, you determine what bitrate you want. Higher bit rate compression leads to higher quality media presentation, but it also increases the minimum bandwidth requirements to properly enjoy your streaming media. Visitors with bandwidth equal to or greater than your media's bitrate will be able to see and hear your video and audio clips with little interruption. If they do not have the bandwidth to properly view the media, they will likely experience "skips" in the sound, or pictures will freeze while the sound continues.
Length of Media Clip
How long your audio or video clips are has a lot to do with how much it costs to stream it. Each second your clips are streaming, an amount of data transfer equal to the bit rate compression is used by the streaming media server.
Number of Times Clip is Accessed
The final factor is simple. Each time your media is accessed by a visitor, it uses bandwidth. The more people come to listen to your audio or watch your movies, the more bandwidth they use. MediaHostingWorks has plenty of quality bandwidth to go around, but it's not free.