Yes, it's different, but not certain skill related, as you assumed.
It actually depends on the total salary. Skill is more expensive if the guy is at 200, than if he's at 20k.
You can check salary calculator at Buzze-rManager.com
Also check this salary formula at http://buzzerbeater.wgz.cz/rubriky/salary-formula. It's old, but still useful to get a feel about what skills are more expensive