Also depends on if they are using magic rack or not. In 9 ball with the magic rack, their % would probably be above 50% with all the packages. In 10 ball it's going to be significantly lower, with or without the magic rack.
They often show the stats for break and runs during the 9 ball US Open. Let's assume that 90+ % of the players that compete are A or better. In the last open the average for all players was in the low to mid 20's on a 9' table.
I think the better question would be, when a ball is pocketed on the break, what is the break and run percentage? A professionals break and run percentage on a typical bar box, with worn slow cloth, dirty and mis sized balls using a standard triangle rack is going to be lower than on a tournament quality table with new balls using a magic rack.