Exactly.
Do a fellowship because you WANT to do it, not because you’re chasing dollar signs. As said, you can find yourself very disappointed. I’m cardiac trained and I absolutely know generalist who make more money than I do.
Think about it like this. Who probably had the better week financially, the guy who did a couple of open hearts and maybe a day or two in cath lab, or the guy who did 3 spines one day, a couple lap choles another day, and maybe a day at an ASC, and the another day of ortho.
As said above, you have to actually do the work to make the money, at least in most private practices. In academics, a fellowship may get you a 50k bump in salary, but academic CV only positions are becoming harder and harder to come by.