If you look at this video from the Google I/O conference and the slides you can see that they show how much WiFi, the CPU, screen, GPS, accelerometer etc. use in a device.
Here is the video and the slides.
A quick summary in case the links go down:
This shows the amount of battery used by the components in milliamps (mA). Another unit needed is milliamp hours (mAh) which is what battery capacity is measured in, a typical smartphone might have 1600mAh capacity. Having just the screen on we can see how long the device may last by dividing the capacity (1600 mAh) by the usage (~100 mA) giving us 16 hours of use with just the LCD screen on.
It's an interesting video if you take the time to watch the whole thing. It discusses wake locks, where an app will wake up the device to update itself, and how they affect battery life which is an aspect I don't usually consider about applications.