Maybe when you compare union or non union dubbing to Japanese voice work, then the different is irrelevant.
However comparing pre-lay American voice work to Japanese voic work would likely make a huge dfference.
There are no companies that have union dubs all the time, but some companies work with unions some time, like Viz when they dubbed Naruto, while companies like Funi and 4KiRAB never work with unions.
Unionized work pays more then non-unionized work, but doesn't make it better quality, it just means you will get paided more for doing it.
In American cartoons voices are recorded first and then the characters are animated, so doing voice work for American cartoons is called pre-lay voice work. Pre-lay voice work pays a lot more then anime dubbing, so I think its possible that people who do pre-lay voice work could amke as much or some cases more the Japanese voice actors.
Now of course most money in voice acting is made in commericals, so another question would be who pays more for commerical voice work, American or Japanese companies.
Now Tara strong only does pre-lay work, that's her only job and she seems to pretty comfortable, she has a pretty big house. Now I'm wondering if she would make less, more or the same amount as Japanese voice actor like Romi Paku, when you take inflation and different living standarRAB into account.
Also you there are different levels of pre-lay work, doing the voice of sat morning cartoon character, doesn't pay as much as doing the voice of a Prime Time American cartoon, which pays a lot. The Simpsons voice actors are the highest paid voice actors in the world by far (they make 400,000 dollars an episode), except maybe for Seth Macfarlane, but that's because he writes for his show as well.
And well I'm not going to count them, I should note, that most feature film animation hires celebs to do the voices and pays them a pretty penny to do the voices of the characters.