It really really varies from fellowship to fellowship. First year of a fellowship like cards or GI is pretty much just as bad as your intern year.
Pay-wise, residents all start between 45 and 50 thousand a year varying depending on geography and it generally goes up by $1500 or so yearly. So a PGY1 resident (i.e. an intern) might start at say, $47,000 and be earning $50,000 by PGY3. Fellowship is generally treated as an extension of your residency, so if you were to do a cardiology fellowship after IM, you'd simply start as a PGY4 (since you already had 3 years of IM training). So a first year fellow coming out of a 3 year residency would earn on the order of $52-55k.
Note: This doesn't include any pay you might get via moonlighting. Now, resident moonlighting experiences are generally limited, but depending on the program people often can supplement their income by 50% in the last year to year and a half. On the other hand, a fellow is also a board eligible (or even board certified) physician in their primary specialty, so they can earn a hell of a lot when moonlighting if they're willing to put in the time. This is within the limits of duty hour restrictions of course, because any time moonlighting still counts towards your 80hrs/week.