Aero guru Chet Kyle has this to say on the topic:
We measured this in the General Motors Wind Tunnel in 1996, and on the track using the SRM crank dynamometers. The lead rider in a 4-man pace line uses about 2 to 3 percent less energy than they would if riding solo.The next in line needs about 71 percent of the lead rider’s power, and the third and fourth riders about 65 percent. See “Racing cyclist power requirements in the 4000-m individual and team pursuits”, Medicine and Science in Sports and Exercise, v31, no.11, pp 1677-1685, 1999. J.P. Broker, C.R. Kyle and E.R. Burke.