Emm, this is a subjective question because it all depends on what you're trying to do. If you're trying to build a cloud environment the requirements for that is far different from having a simple MP3 server.
Most co-lo's I have seen charge according to footprint and/or power consumption. There are some that will charge on a per rack basis that would include the environmentals and power. Some will separate and itemize. I would suggest looking at as many co-los and pricing schemes as possible.
One thing I can say for certain is that although servers may be specc'd for a certain range of power consumption they almost always never consume that much and in most cases not even half. So if a server is rated for 400 watts of power most times I'll see them sitting around 150 watts under normal operating conditions.