Some of these trades do have apprenticeship program's in their unions and at least in my city in the licensing requirements. To be a journeyman electrician you are first an apprentice for 4 or five years, and at least in union shops your pay reflects your (theoretical) ability.
So a fifth year apprentice make something like 80% of a journeyman as they should be capable of 80% of his tasks; while a first year apprentice pulls in less than half as he likely knows little of the trade.
Vast majority of these apprenticeship programs first require that you have a 2 yr degree from some technical institute though, and there's very few that will hire you based on high school grades and agree to pay for your education with the exception of some power companies. Seems if you're crazy enough to be a powerline tech they will pay to train you.
So a fifth year apprentice make something like 80% of a journeyman as they should be capable of 80% of his tasks; while a first year apprentice pulls in less than half as he likely knows little of the trade.