That's impossible to quantify. A skilled player will simply do...better. DPS isn't the only hallmark of skill - proper management of cooldowns, of getting out of fire, of doing your job (control of adds, for instance) correctly are all signs of a good player.
Greatly depends on the class. Some classes are much more, eh, 'noob friendly' than others, being more conducive to poor play. Others require much more micromanagement, attentiveness, etc.
Really there's no magical formula for how well X player does against Y player, even on target dummies. There will always be someone better, always be someone worse. Some players have handicaps, and some players have almost mechanical precision with how they play their class.
Truth be told, it's all about how well you WANT to play.