If you are refering to private practice surgeons, (and i can only comment on how it works in australia) they recive pay for service. That means each operation the surgeon can charge their fee. The more operations, the more fees and hence the higher wage.
If for example a neuro surgeon charges 10,000 for an 10 hour operation (prolly longer), but a cardio surgeon charges 5,000 for a 3 hour CABG (coranary artery bypass graft) then the cardio surgeon can perform more operations in the same time and hence make more money than the neuro. Hence a neuro surgeon may make more money per operation but because they are very intense and long duration they cannot do as many as a cardio surgeon.
The classic case is with elective orthopeadics. Now im sure this is an isolated case but we have a family friend who is a top private orthopod and he did $50,000 worth of operations in 1 day - mainly knee recons and knee arthoroplastys.
I would assume in public hospitals were doctors are on set wages then the neuro surgeon may get paid more.
However my personal opinion is you carnt live without your heart but it appears many people can live without a brain.
