You can save an enormous amount of energy while drafting. Most especially the further back you are in the line.

Case in point...

This past weekend I was riding in the number 4 slot of a paceline. While I was putting out 210 watts, the guy who was pulling was hitting 300 w. That is not a small difference by any stretch.

Most things I've read suggest that depending on what position you ride within the paceline, you can save as much as 35%.