Simple answer. Whatever YOU are willing to pay for one. One can stumble on a 60 incher anywhere and pay whatever that outfitter charges. You can have a PH find one in his area and you may never see it. As you know big kudu are cagey. They dont get big by being stupid.
I shot a 62.5 in. Kudu in Namibia this year. The trophy fee was $1300. He charged $300 more for Kudu over 55 in. We spoted the bull and it was with some cows and we thought they went on into the bush but the cows came back and the bull followed them. They jump a small fence about 220 yards away and when the bull came to the fence he broadsided before he was going to jump and that is when I shot him. He made it over the fence and dropped a few yards on the other side. There was a high fence by the road but I do not know about the other side. The place was 55000 acreas in size and the fence next to Botswana would'nt stop to much. I actually did not know how good of kudu I shot until we measuered it. If Johan would have said that a 60in bull I may have missed him.
I'm sure there are some areas of Namibia, Zimbabwe and Mozambique that meet the 60 inch limit. Tough hunt sounds interesting. Means you might also have to settle or go home empty. The different between 59 and 61 ain't much. Thickness of horns, depth curls, and length of horn....anyone really good at judging? To me if one looks SUPER and gets the heart pumping is a SHOOTER and what it scores...matters very little.