He says that it wasn't money motivated and Carter seems like a pretty straightforward/honest dude. Seems like the team over at Evans/Promark just was a bit easier to work with and treated him better. Which goes a long way.
All that being said - his drums sound great with the new heads. I'm sure he could make a sound percussion kit sound great though just because he is that good of a player. Evans is lucky to have him.
It is rare that any Artist gets any money in endorsement deals . Most Drum Companies cannot afford to be paying their Artists . The general practice is to offer support for Clinics and with reduced or rarely free gear . The other carrot they offer is advertising. I doubt Evans outright gave money in payment to Carter .
Many time the reason behind people changing relationships is because of Resource changes in the A&R Department . Most drummers at Carter’s level can make anything sound acceptable and the relationship with the A&R is important .
And the discounts are generally the same, manufacturer-to-manufacturer. Especially for two "big" (for the drumming world) companies like Evans and Remo. These types of moves are very rarely motivated by money, and usually motivated by relationships. Tour / international support can be another reason someone would move, but again, given the reach of the two companies involved, I don't see that being an issue.
I'd assume that Carter has developed a great relationship with D'Addario due to the experience prototyping his signature stick, and decided to expand that relationship to include Evans as well. We'll see if he starts using all Planet Waves cables as well