The NIH is a government agency, and that means its scientists will have to scrape by on salaries pegged to the government pay scale. As an entry-level lab tech right out of college, you might make $35k each year. Staff scientists with a Ph.D. and several years of training will usually bring home closer to $75k to $100k. If you are lucky enough to be your own boss and run your own lab as a principal investigator, you might be making a hefty $125k annually—and that's nothing to sneeze at (unless you aspire to join the top 1 percent—but that's another story).
While there are paychecks to be had at the NIH, some trainees at the NIH work for free. They say it's that whole "invaluable training" thing, the experience that makes your résumé shine. And hey, if a summer internship gets you into the college-of-your-dreams, you probably won't be missing the $1,000 the rest of your friends made whipping up Egg McMuffins that summer.