There is no bump in pay for fellowships. A first year neonatology fellow or first year cardiology fellow will earn the exact same as a 4th year general surgery resident at the same sponsoring institution.
At most places, yearly increases in salary are automatic, so by the time you get to the 4th or 5th year, you're making closer to $54-56k, depending on where you started. However, depending on where you did residency and where you do fellowship, it's entirely possible to take a significant pay cut, though it'll probably be made up in the difference of cost of living (though it may not feel like it).
The one difference that may enhance fellows' salary - particularly those coming out of IM and Peds - is moonlighting. Once you're licensed, you may be able to find locations (acute care/doc in a box), that will pay you well for taking care of coughs/colds/ear aches and gastro cases. Of course, whether your fellowship allows moonlighting either by policy or in the amount of free time you have (critical care fellows may have difficulty in finding enough time) is a very different discussion. Also, it's a little tough to find moonlighting opportunities for surgeons...