Quote Originally Posted by soulkush View Post
For runes, it shouldn't be a set chance. I fairly certain it scales to players ingame
There's a set chance per rune for each entity that may drop that specific rune, and the number of players in game multiplies that % in a kind of diminishing return as the number of players increase, all that doesnt include bosses which have a set number of items to drop (example, mephisto usually drops 5 items all the time, so more players wont ever change the quantity of items dropped).

Ask @Game , he probably knows a lot more than us about this :P