It's going to be different for every vehicle, plus the largest variable is how long the bed is. I found the following article:
http://www.sema.org/Main/SemaOrgHome.aspx?ID=52546
Which describes pretty much exactly what you speak of. The original article was a VERY lengthy writeup and included pics from the wind tunnel and everything. It was a standard cab pickup with something like an 8-foot bed, which is as long or longer than you typically see on trucks and "sport" trucks with 6 and 4 foot beds. It's been speculated since then that the 6 and 4 foot bed trucks wouldn't see the benefit from lowering the liftgate.
Still, the findings match what you'll see in the Craftsman Truck series, as Troy said, where they run with a capped bed for maximum aerodynamic efficiency. If you look at the link above, a simple bed cap provided almost a 12% decrease in drag, which is substantial. Also, if you were wondering what the "new cap design" was that resulted in a 30% decrease, it was basically a camper shell that started at roof height and tapered all the way down to the top of the liftgate, giving the truck more of a teardrop shape. I don't think anyone actually sells such a thing. A full size campershell would likely increase drag from the other articles I've read.
Revelation About the AeroDrag of Pickup Trucks
I was left concluding that the best way to minimumize drag on a pickup truck was to put tarp over the bed. Either that, or put a cab in the bed.
