I found a passage in Tim Noakes book about uphill and downhill running. He states on page 61 (newest Lore of Running book)that C.T. Mervyn Davies studied energy expenditures and found that every 1% of uphill running required 2.6 ml of O2 more, equivalent to .65 km per hour reduction in running speed. Addtionally, downhill running required 1.5 ml of O2 less for every 1% change or an equivalent speeding up equivalent of .35 km per hour.
A side note, on the previous page he stated that research showed that anyone running slower than 2:21 marathon pace (about 5:24 pace, I think) does not benefit from drafting. Interesting!