CPU usage also depends on what features are enabled and how much work the addon is doing. To use Recount again, it may use 0 CPU cycles when sitting there doing nothing, increase when in a party of 5, more when in a raid of 10, more for 25, more for AV, etc.
Also, judging CPU usage depends on the computer's processor and how much it can handle. My new computer can take a whole lot more being thrown at it before it shows an impact on performance than your lower end machine.
I hope you don't feel like your head is being bitten off in this thread, because that's not really the case. It's just that this is just not feasible.
I suggest installing an addon to look at performance stats (I prefer OptionHouse, myself - you can use it to enable/disable addons, too) and just try different things until you find something that you like and can use.