Gwen's net worth is in the same neighborhood as Ariana's (180 - 150 mil) and more then Kelly and Blake's combined. But you would never advocate for paying Gwen more then Kelly and Blake combined in salary. Voice coaching salary is based upon what the producers think you add in terms of monetary value to the show. That is best, but not exclusively measured in TV ratings.
The argument that I think you're trying to make is that Ariana has the most marketability outside of the voice, so her time is the most valuable, in that she could be doing other things with her time, and making more money then the other coaches. So you're compensating her for not doing those other things.
From a voice standpoint though, Blake is probably the most valuable coach.