I think you need to measure it yourself. After 1,000 miles of riding with my Garmin Edge 305, I can confirm that on hilly rides I average about 5 - 9% faster than on flat ones.
I live near a rail-trail that's very nearly (but not completely) flat. It drops about 200 feet in 11 miles one way, and I always go out and back. It's flat enough that there is no coasting - you pedal all the time.
My other rides are loops in slightly hilly central Massachusetts. I may slow down to 9 - 10 MPH on the uphills in spots, but hit 35 - 37 MPH on the downhills. If I went up a half-mile hill at 10 MPH I would be going a heck of a lot faster than 20 MPH coming down that same hill! All courses are loops ending at the starting point.
On downhills you also have the opportunity to coast and rest a bit, too.
I have ridden about 600 miles of hilly courses and about 400 miles of the flat rail trail. My overall flat course average is 16.4 MPH, and my overall hilly course average is 17.5 MPH. So I'm almost 7% faster on the hilly rides. I suspect that's small enough to be dominated by individual style/performance so I would guess that some folks are faster on the hills and some are faster on the flats.